The general recommendation is to get one of the common rebreathers -- the ChOptima is fine -- and dive the backside out of it for a good 200 hours or more. The simple progression is then to do MOD2 (normoxic trimix to 60m/200ft or 70m/230ft), then dive the backside out of that.Yea I'm a million years away from it. Just like to buy once cry once. If the Choptima can be utilized in other ways as my diving befokes more technical then great. Currently I just want to travel and hang out in the 100'-150' range for as long as possible. However I know that'll lead me to other adventures and want to make sure whatever I buy now can still be a functional part of my future kit.
The fact a buddy already dives the choptima makes it ideal. Maybe we can convince each other to buy matching back units later lol.
Progressing beyond that requires a lot of dedication, practice and skill, as per the MOD3 course (circa 100m/330ft, hypoxic mixes, etc.)
Back to the early days; diving CCR is a lot more involved than open circuit. Specifically there's a lot more prep and planning, plus a ton of different skills to constantly master including bailouts, flushes, failures and more failures. The hard skills are doing ascents in open water and keeping your buoyancy spot on at your decompression stops -- the 6m/20ft stop is surprisingly hard until one day it all clicks into place.
Diving a rebreather is really pleasant and worth all the extra effort for the freedom it brings.