This is built-in on the latest versions on Android. Open the Settings app, then Battery -> Battery Health -> Battery optimisation and choose Limit to 80%.

Yes, but it requires root: AccA.

you could replace your battery every 6 months but make sure you get genuine one (from official service, no clue if they still sell for this model) not some shady chinese aftermarket option

It appears very difficult to do that without rooting the phone. You could instead have the notification trigger an external switch that disconnects the charger, or just put the charger on a timer. I just use my phone's timer alarm to tell me when to unplug, and that has worked pretty well.