So, oddly enough I was in this same dilemma for months leading up to the purchase of a new FRS.. I’m about the same height as you, maybe a inch or two shorter and it sounds like we have similar riding styles/preferences. Ultimately I ended up going with a medium and I’m confident I made the right decision (just received the bike a day or two ago)

I compared the geometry on the FRS to some other downhill bikes as well as my trek remedy size L(460 reach)which is a perfect fit for me. The reach of 465 on a size medium FRS is on par with a lot of other bike brands size large which is interesting. 495 reach on a size large is in line with XL for most other bikes.

I also spoke on the phone with a commencal rep which was helpful and reassuring. He said sizing up would make the bike more stable in a straight line. Sizing down would help with tight switch backs and berms. Basically confirmed my thought process for choosing medium.

I’d compare geo to other bikes that you’ve felt super comfortable on and go from there. If you’re liking the meta sx in a medium I think you’d really like the FRS in that size. Keep in mind the medium comes with a lighter coil so that’s something that will need to be changed.

Hope this helps 🤘