AccuBattery and AnTuTu. Make sure to run Android 10 so you have access to Mah readings. If you're using AnTuTu, charge at least 80%, otherwise the reported values will be ridiculously inaccurate. AccuBattery delivers somewhat reliable values for charges 30% or greater.
As for battery replacement I have no idea. Maybe you can find an original battery that has been cared for over the years. I would not trust 3rd party batteries, but at this stage of its life cycle maybe it's worth a try.
Hi everyone, before anybody goes ahead and replaces the battery it is worth installing a good android 9 ROM like Los 16 or mokee 9 because the difference in how long the battery lasts compared to android 10 and 11 ROMs is big. So it is worth checking before you think your battery is on its way out.