not sure if you're still looking but I managed to install LineageOS 18.1 GSI on my BV9900 just this week. Not perfect (I have no fingerprint, screen wakes up when it shouldn't, TWRP cannot mount /data, Google Pay not working...) but seems mostly okay. Will do some more work to see if I can get at least some of those things working.
As for rooting the stock system, I believe the unofficial TWRP should work - the recovery worked for me (the SP Flash method with some hacking).
As for LOS, I believe this is the best instruction set for this phone. As I mentioned at the beginning, I got the LOS 18.1 GSI working eventually. I even got the GAPPS working in the end, flashed via TWRP after some resizing magic, and registered with Google accompanied by repeated wiping..
Good luck and let me know if you have success (especially with the things that don't work for me, pretty please).