Batteries are weird. Once they get into the 80s when it comes to maximum battery health/capacity they need to be replaced.

I had an older iPhone 8 battery replaced at a battery shop near me before I sold it and it allowed me to ask for another price. I’ve got an “older” iPhone 12 that I gave to my mom and I’m planning on passing it down to my little brother this year but before I do I’ll probably have the battery replaced. But this time I’ll have Apple do it. It’s possible to do it myself but the price to effort ratio isn’t there, especially since I care about keeping the water resistance.