I think that their number are overstimated, at least after the prothean war.
They attacked the prothean when they were immensely more advanced than this cycle (they were one step away from building mass relays, they can blow up stars, they were at war with super-advanced AI javik believes that we are primitives).
So, why attacking this cycle so early? (the attack was probably scheduled before the rachni war)! Porbably because they know that they can't afford another war like the one against the protheans.
They showed inability to completely eradicate the prothean: scientist on Ilos, Vendetta, the beacon on Eden prime, Javik, the Mars archive, the crucible... the work wasn't done properly.
They showed to fear losses and to have the numbers to fight only on a limited number of fronts: they try in every way to gain access of the Mass relay (Sovering, Arrival: destroying a system one by one seems vital), the don't attack directly the Citadel (they use Cerberus infiltration) and they don't attack all races at the same time, allowing cooperation and aids between them.
They don't the strenght to sustain an effective "total war strategy" in all the galaxy.
Finally, if Shepard choose refusal, they will defeat this cycle but they won't be able to complete the next cycle (they are no longer a threat)
So, they could claim themselves invincible and countless, but they behave exactly as if they are not.
They are very powerful, far more powerful than all the galaxy combined, no doubt, but they truly seem to have suffered (in the previous cycle, and in this one) relevant, possibly irrecoverable, blows.
Well, you managed to be wrong about nearly everything. That's impressive, in its own right.
"can't afford another war like the one against the protheans"
You mean a total curbstomp where they lost so badly that a single Prothean surviving to the modern day was considered miraculous? Yeah, the Reapers lost sooooo much. They could have afforded a war even worse than that without difficulty, that was nothing for them.
"They showed inability to completely eradicate the prothean: scientist on Ilos, Vendetta, the beacon on Eden prime, Javik, the Mars archive, the crucible... the work wasn't done properly."
Because of trickery, and luck. No part of this had anything to do with Proteans being awesome, it was luck and trickery. Ilos was only not found and destroyed because the data about it HAPPENED TO GET DESTROYED BY COINCIDENCE BY THE REAPERS. If not for that LUCK, Ilos would have been destroyed without fanfare and there'd be no hope for Shepard's cycle. The trickery was because they were decent at hiding things, arguably better than they were at destroying. So yes, I give credit, Protheans are going at hiding. But that doesn't mean good at fighting Reapers, I'd be surprised if the Reapers lost more than a destroyer or 2.
"They showed to fear losses and to have the numbers to fight only on a limited number of fronts: they try in every way to gain access of the Mass relay (Sovering, Arrival: destroying a system one by one seems vital), the don't attack directly the Citadel (they use Cerberus infiltration) and they don't attack all races at the same time, allowing cooperation and aids between them."
Only a limited number of fronts? You're kidding, right? By the end of the game, the entire galaxy is their front. They're attacking every single world equally by that point, their numbers are darkening the sky of every world. They're carrying out over a dozen fronts at the same time, and making it look easy. Yes, they would prefer to make it even easier, but that's simply because even AIs would prefer an easier job to a harder one. Besides, tactics are tactics. Why wouldn't you want the tactical advantage to controlling the relay network, no matter how superior you are? They don't attack the Citadel directly because it's important to their entire strategy. Or have you forgotten that it's the key to the typical Reaper tactic of using it as a trap like they did with the Protheans? Of course they'd rather not damage it, or they'll have to rebuilt the damn thing and waste even more of their time and effort. You act like they're cowards, but it's the exact opposite. They lost their relay network and Citadel trap advantage, so what did they do? Decided they didn't need it and just showed up and started kicking as anyway, without hesitation. That isn't being scared, that's being without fear at all. If they acted like you said, they would have attacked the Citadel first since they would have decided a bit of extra work and effort is worth not the risk of losing. The fact that they let the Citadel be is a sign of their confidence, not fear. Cerberus was used because they might have been able to take it with far less damage than a bunch of Capital Ships showing up and just dumping a few million husks on the station, Cerberus could take it with much less collateral damage than Reapers could (because being gentle isn't something Reapers know how to do). Of course, when it's all over and they know they've basically won, they do take the Citadel. But that's another sign of confidence, they saved the tactically most important target until it wasn't even that important anymore, they took it only after they no longer needed it for their trap. Hell, they even did it without much damage after all (which is extremely impressive). The only reason they even bothered to capture it when they did was to be the bait to lure the entire galaxy into one massive trap that was the final battle, because they knew that everyone would try to get it back.
On that note, something else. They purposefully didn't shut down the relay network after taking it. Earth was a trap, the plan was to lure the entire galactic fleet and the Crucible to Earth, so they could bring all their enemies to them and save them the trouble. They could have shut down the relay network the second they got the Citadel, but they didn't. Them doing that would have been the "coward tactic" you seem to think they were using in ME3, but instead they decided to intentionally let the galactic races have an advantage and then purposefully let themselves get hit by the entire combined forces of the whole galactic community. Instead of shutting the network down, they left it up and waited for the most powerful anti-Reaper fleet in history show up for a total war offensive against them. That is NOT being afraid of losses, that shows a ludicrous amount of confidence. They know that the biggest fleet they've ever fought is coming, and they let it happen because they figure it will simply be more convenient and time saving for them. I repeat: They let themselves get hit by the biggest fleet ever just for the sake of saving time, because they were so confident that they'd win that they considered destroying that fleet easier than hunting them all down individually. This is the absolute opposite of what you said it was.
"They don't the strenght to sustain an effective "total war strategy" in all the galaxy."
Hm, is that why they left their asses hanging out for the entire allied fleet in the final battle above Earth? Is that why they let the largest fleet ever assemble to destroy them, purposefully left the network up so said fleet can reach them, all for the intention of battling and winning the biggest total war strategy that the Reapers have ever been faced with? So they didn't have the strength to do so and they knew it, yet they let it happen with full knowledge of what was coming? That makes no sense. They would not have let that happen if they were actually afraid of battling the entire galaxy at once, since that's exactly what they did in the end.
"Finally, if Shepard choose refusal, they will defeat this cycle but they won't be able to complete the next cycle (they are no longer a threat)"
Only because of more trickery and luck. This says nothing about the Reaper's power, it just has to do with it being hard to know where ants stashed every crumb in the galaxy (and every crumb could tell the ants how to build a machine to kill your entire race in one hit).
"So, they could claim themselves invincible and countless, but they behave exactly as if they are not."
No, they behave exactly as if they are. Invincible and countless beings let themselves get hit with the largest enemy force ever for the sake of convenience and time saving, invincible and countless beings save the war ending tactical capture of the Citadel for last (as in saving until after they don't even need it) because they know they can win even without owning the relay network, invincible and countless beings wage over a dozen fronts at a time without regard for fuel lines or anything and still win.
"They are very powerful, far more powerful than all the galaxy combined, no doubt, but they truly seem to have suffered (in the previous cycle, and in this one) relevant, possibly irrecoverable, blows."
What? Maybe 3 destroyers in the Prothean war, tops? MAYBE a capital ship, if they got unlucky? They barely lost anything against the Protheans, something even Javik expresses. The Protheans were about as threatening to the Reapers in terms of military power as a a few dozen infantry against a nuke.