Could overnight charging damage my iPhones' battery?

Having an iPhone over 80% for long periods can shorten battery life. In Settings > Battery > Battery Health & Charging, you can enable "Optimized Battery Charging". This will limit overnight charging to 80% for most of the night, then allow charging to 100% for your normal wake time. If you don't need the full 100% you can set it to never go over 80% (if your iPhone is model 15 or newer). Also, using a low power (5 watt) charger will extend battery life. It will be slower but that doesn't matter for overnight charging.

I have an iPhone 14 that is nearly 2 years old. I use a 5 watt charger and rarely let it go much over 80%. (I have a shortcut that beeps when it is over 80%.) The battery health still shows 100%.

No, in fact Apple recommends charging overnight. The charging circuit will shut off once it reaches 100%. You cannot overcharge the battery.

No, it can't hurt the battery at all and is in fact the absolute best way to charge your iPhone.