Should I charge my Apple Watch Series 10 to 80% or 100%?

The watch will recognise your charging pattern over time and adapt the charging process accordingly:

About Optimized Battery Charging on Apple Watch - Apple Support

I let my Ultra 2 charge every 2-3 days, when it reaches around 20%.

The watch always charges to 100% automatically, optimised charging is enabled, because it learned, that I need it right after that again.

Basically, the watch learns how you use your watch. So if you usually wake up at 7 am, it waits until 6am to charge to full. And this works on different days. It connects with your wake up alarms set in the sleep schedule too so it knows when to charge to full, preserving the battery.

The algorithm aims to ensure that your watch is fully charged by the time you remove it from the charger. 

If you need your Apple Watch to charge past 75-80% level before it's scheduled to do so, you can make it charge to full manually.

1. Keep your Apple Watch connected to its charger.
2. Tap your watch to show the charging screen, then tap the circle with the green or yellow charging icon
3. Tap Charge to Full Now

You can also pause Optimised battery charging for a period of time on 9/Ultra 2 or later.