The other thing is the certainty that you just can't defeat the Reapers in a conventional war because they are just too powerful. I say that is complete rubbish. The U.S. had superior military power in Vietnam and couldn't win there. The Soviet Union lost in Afghanistan. You can beat the Reapers, you just don't take them head-on. One way is a war of attrition, but would require heavy sacrifices. You could use the Trojan horse strategy. Leak where the Crucible is being built, but it's a fake location. Evacuate people from the planets in that system beforehand. Lure as many Reapers into the system as you can, engage them for a short time and then run for the exit. After the last ship gets out ahead of the Reapers, a timed explosive blows the mass relay and destroys the whole system and the Reapers with it. There's always viral warfare. The same technique the Reapers use to indoctrinate can be turned against them. Infect the Reapers, change their programming, turn them on each other. The writers went out of their way to make it all so hopeless and there was just nothing anyone could do to stop this menace.
Both of your examples - Afghanistan and Vietnam - are examples of a distant power with public-opinion and financial problems attempting to set up a loyal, stable local government that could be left to its own devices and withdraw. They were not battlefield force-on-force victories - both the American and Soviet militaries were virtually unchallengeable in that sense. The Reapers had no such problem. They were not out to set up a puppet government that could run things on autopilot, they were out to eradicate all higher organic life. Vietnam and Afghanistan are irrelevant in such a context.
"War of attrition" is a label often applied but almost never understood. Fundamentally, all combat is attritive: both sides lose troops, and both sides try to achieve a favorable kill ratio because running out of troops first is a bad thing. The label "war of attrition", however, is most popularly used to describe scenarios with minimal operational or strategic maneuver, in which soldiers are sacrificed in high-casualty operations in the hopes of generating some sort of advantage somehow. It is used
in lieu of alternative, low-casualty options, with the implication being that the only way to give an enemy any pause at all is spending lives, bar none.
Leave aside that this popular take on "attrition" is a fundamental misunderstanding of
the original articulation of the strategy. It is difficult to see when such a strategy has ever actually worked in doing what it intended historically, and it is even harder to see how it would be relevant against the Reapers. Javik, for example, claims that Prothean fighting against Reapers focused on attrition, which supposedly slowed the Reapers down (how? why?) and gave Prothean forces time to regroup. Effectively, the Protheans inflicted shattering defeats on their own forces in order to give their ruined military an opportunity to recover from
their own insane strategy. Rinse and repeat. This is not a recipe for survival, let alone victory. Notice that Javik never mentions the hope of inflicting higher relative casualties on Reaper forces (probably because it was not possible): this makes Prothean strategy not "attrition", but suicide.
A mass-relay explosion trap for the Reapers is a moderately interesting and potentially useful (if hardly new) idea, but there are problems. The number of Reapers that would actually engage a Crucible under construction is questionable. So is the presumption that they wouldn't see such a trap coming, or that they wouldn't simply enter FTL travel to escape instead. Such a device would also only work once, if at all. Any idea is better than no idea, but
one idea isn't enough to win a war. It doesn't change the fundamental calculus that the Reapers outnumber and outgun and out-tech the civilizations of the galaxy, and that they cannot be crippled politically or negotiated away from their path of extermination.
Also, such a plan would only work if the Crucible were a viable weapon to use against the Reapers (otherwise, why would they attack it?). If that's the case, that sort of devalues your CONVENTIONAL VICTORY AT ALL COSTS message.
Indoctrinating Reapers, while also a potentially useful idea - as, apparently, Sanctuary made clear - also doesn't really qualify for conventional warfare for any definition of "conventional" that I've ever seen. It just replicates the Crucible's function without replicating its form.