GSam has settable alarms for low below% and above%, and selectable notification tones for them.
But rooting with Magisk is even better - you can run Magisk Charging Switch and set the maximum charge amount (even with the charger plugged in it won't charge until it reaches the minimum current you set it to start charging at). So you can run really safe - 80 40, you can keep the charge at almost a single point, say 75%, or you can destroy the battery by setting it to 100 0 (not using it at all). But if you don't want to walk around with a long charging cable, MCS and GSam make it easy - plug it in and unplug it when you think about it, but if it discharges too far when it's not plugged in, GSam reminds you.