As already mentioned here, you could use "mode and routines" (alternatively apps) to trigger certain events (e.g. sound) when the battery level is reached that you can manually unplug. Theoretically you could also work with a timer on the charger. But this is all complicated and no good solution...
I don't have a Galaxy A21 with me. Unfortunately, the energy saving and sustainability options of many devices are extremely modest... In my opinion, some of them even don't function or seem to do something.
However, I would put the main focus on generally trying to reduce battery consumption so that you don't have to charge as many times. I wouldn't put much emphasis on 85% or 80% either. That shouldn't make a big difference. The value alone is likely to be quite arbitrary; a proper investigation would not result in smooth values
