wrt. 3. 4. and 5. using havocos on op6 for 6 months now and never experienced those. maybe a clean install with latest oos firmware will help
1. do a factory reset in twrp
2. install as described in
https://forum.xda-developers.com/showpost.php?p=80220358&postcount=5053
also
did you ever try without doze and naptime for a while? im not using those and have no problem with uptime. 48h standby. 36h when using moderately.
wrt. 1. in 6 month this happend maybe 3 times only
using blu spark kernel and underclocked the cpus to 1600/1056. using magisk with gms doze.