Move all FX off the main audio thread

What do you mean by "main thread" ? Each track uses it's own OS thread, anyway. (Moving an audio stream from one OS Thread to another takes up additional CPU resources.)

REAPER has a main audio thread that interacts with the audio device as I understand it - I may not be expressing it correctly, but that is the idea. When plugins are on this main thread, this is when your RT CPU shoots up like crazy and can result in lot of snaps, crackles and pops in a loaded mix. Resourceful users have figured out that you can create several layers of tracks leading up to the main mix bus and move FX to those individual tracks. RT CPU drops through the floor and you can use a lot more plugins on your mix. Acustica and Nebula stuff are really good examples of things that can really load your session and cause RT CPU problems, unless you move them to other tracks. The result is that CPU load is spread and your main audio thread is light as a feather. No more tell-tale CPU strain staticy crackles happening at random times. I know this to be the case. I do it all the time myself, as do a few other users.

The thing you want is Anticipative FX processing on Master Track and Monitoring FX. +1

The "main audio thread" you describe is basically the one which doesn't benefit from Preferences > Buffering > Anticipative FX stuff. Big words just to say that the FX on Master and Monitor FX are processed in Real Time, so no matter how big we set that value, every single one of those FX in these places are in the real time path anyway. But like you, i feel this doesn't have to be like that in ALL cases. I mean, we all do the "submaster trick" just to move EVERYTHING to the Non realtime path... That said... It would interfere badly with record armed tracks (which are always in realtime path, so they couldn't pass via the master track)