I've replaced battery for G2 for quite cheap in SLS, time taken 20 minutes. It has been non-issue for me. Battery can be original (more expensive) or reputable replacement (slightly cheaper). Prices are .... uhhh .... cheaper than buying original OEM removable battery.
For god's sake charging overnight is OK. Each phone has a charging profile which gives a charging curve, when it is at 100% the charge change to trickle mode (very small charge for maintenance). The charging profile controls a chip they call a charging controller, which communicates using data line (for USB C) with the charge controller on the charger. It is by design not intended to overcharge. If you use a custom ROM then you better know what you're doing because the charge curves may have changed for faster but unsafe charging. If you use a 3rd party charger you better be confident that they have a properly profiled charge controller onboard. If all is proper then you shouldn't worry. You can even monitor your charge amp using a simple app like
Ampere, you'll see the charging current go down after 85% and almost zeroed out at 100%.