Can I get my battery health back at 100% without servicing

The only thing that can move the percentage up is doing a Force Restart, since that resets the calibration being displayed. But in general terms, there is no way to reclaim lost youth, be it a battery or real life.

https://support.apple.com/en-us/HT201412

Hello

So I think it would be best if you refer to Batteries - Maximizing Performance - Apple

Hope this helps