3.2 Channel with upgraded L/R or 5.2 channel?

YES!  Keep the 5.2 (2 subs)!  The ability to have various forms of "decoding" stereo into multi-channel sound will let you be "in the room" for many kinds of music: jazz clubs, concert halls, even county fairs, etc.  I have two multi channel setups in my home, Sporting events can put you in the stadium with the crowd. Great fun and a great distraction if you are hit with Covid! (Had a 7.2 system, but didn't think it added enough to be worth the expense. SACDs are 5.1 anyway.) Happy Listening!

Unless you've got a humongous room, those REL 212s already have your room well-pressurized with bass energy; adding more subs would primarily increase ambience, so you don't need to go big to get the job done.  Echo the recommendations regarding the Elite receiver.  Good in its day, but now....

Agree with everyone else. Keep the 5 channels absolutely! I too run 5 channels like you but mostly listen in 2 channel. But every time the wife and I watch a movie we really appreciate those surrounds!
 

How about this; add 2 more REL subs? I run 4 SVS’. And whether it’s movies or 2 channel those other 2 subs just added so much more to the experience. I’m not familiar with the Elite unit and whether it has 2 sub outs (via RCA), but if not I would agree upgrade the Elite unit to maybe a Krell Foundation 4K and 2 more subs. This is assuming you would use the RCA and not XLR for your subs. I’ve always gone RCA with my subs..

Get rid of the Pioneer Elite receiver  and get a dedicated atmos/object based audio processor (no amp sections shall be in the same chassis) like the Marantz AV10 or higher, depending on what your budget permits. It will let you enjoy the atmos music albums from so many artists, i.e, you should be able to get to a multichannel rig that's not just for movies or games.

I spose if you got money to burn you could upgrade your pioneer avr  to something like anthem or arcam (depending on what you get you might need another amp)--it might sound a little bit better than what you got, but most folks wouldn't notice the difference.

Those extra channels make a world of difference. Dropping from 5 to 3 would be huge reduction in the home theater experience. Turn off the rear surrounds for a while.

My primary interest is two channel audio, and I am fortunate to have two systems (see my profile). I used to own Olympica.

Either upgrade the left and right channel… I assume you are using those only for two channel audio or leave things as they are.

Long term, if there was a way to separate the systems you would be doubly rewarded. I find that overall the quality level of home theater is different and fine nuance not important since the video is so distracting. You could plan to extract the best components for your two channel and replace with home theater level components.

Looks like you are looking for an excuse to change a top notch system, just because.   You have gone past the excellent equipment and already invested in dubious tweaks.   Buy more music, more movies. Leave your system as it is. 

+1 @loomisjohnson I’d sorely miss the immersive experience provided by surround speakers with movies, and your 2-channel setup seems more than fine. 

 you could always pay more and get better, but your speakers and other gear are really nice and 5.2 is optimal for movies--i wouldn't change your system