Well, I'm going to start by repeating that I don't necessarily agree that it being monolithic is necessarily a problem right now.
The immediate thought in my mind would be all of the federation logic. That's where all of the instances seem to be lagging behind, and it seems the common fix is "just increase the workers to one billion". Apparently that does something meaningful, but the developer in me wants to know how a few cores can put so many workers to use.
Spinning federation off into a microservice means you could deploy it on something like Cloud Run or AWS ECS, and have it autoscale as the workload demands it. Seems like a pretty prime candidate to me.
I'd be less concerned with memory (of which Lemmy seems to use very little), and much more concerned with CPU core count. I touched on it in my other comment, but I don't understand how a few cores is supposed to handle the ridiculous number of federation workers people are setting their instances to.