You can't naturally evolve the lore surrounding the Mass Relay technology without completely suspending disbelief and retconning. The Reapers couldn't get their OWN TECHNOLOGY to work on that level, otherwise they would've just installed mini-relays or whatever into themselves so they could jump back and forth between galaxies, dark space etc easily.
If over millions of years the Reapers can't advance the mass relay technology, their own technology, to that point - I'm not buying that humanity or any other civilisation could do it in a few hundred years, especially when all of their technology is based on the Reapers' technology.
This is an incorrect assumption.
Reapers didn't need to evolve/upgrade. They were perfectly suited for their task.
Getting Relays into Andromeda is easy and has nothing to do with bringing the Reapers back. Just retcon their origin as being extragalactic to begin with, communicated to the Leviathans/Reapers Contact style. Not the prettiest solution, but it gets the job done, and it doesn't invalidate the trilogy or the distance moved from it.
>tag-line is "Explore a new galaxy
>can only explore a single cluster
Or maybe they're pacing themselves this time. How did going straight for 'here's an entire galaxy and related galactic threat" from the get-go work out last time?