You should maybe occasionally check the backside. My wife has a 7a and after battery becoming much worse within a matter of weeks. The backside, as it turns out, was bulging and the battery was the culprit. Maybe it's just slow degradation for you, my pixel 6a also barely manages a day anymore.

Ifixit will sell you a battery. I have one waiting for when my pixel starts getting fatigued.

Maybe a too personal question, do you use graphenOS on this pixel ?