Repairing water damaged devices is always hit or miss. Some phones recover fine, even with no intervention. Others require hours and hours of work and replacement parts and they still remain unstable. It seems you have the latter.
Most microsoldering repair techs no longer offer to “repair” water damaged devices for this reason, they can be inherently unstable.
I would remove the logic board, connect it to a known good, preferably non water damaged, lightning flex and battery. Then connect it to 3uTools so that you can see if it is connected. If it cant maintain a stable connection in this condition, then you should consider cutting your losses. The water has probably seeped inside the pcb sandwich and this is what’s causing your issues.