Hello!
You don’t need a 100 Watt charger. It’s overkill. The maximum wattage you need is a 20 Watt wall adapter. Using a 100 Watt charger won’t make your device charge faster, since the iPhone’s “charger” is located inside the device, and only draws so much power.
You shouldn’t need to follow the 20-80 rule. You should charge your device to 100% each night, and you can charge it whenever you want to during the day. It’s actually not good for the battery to let it drain to 20% or lower if you can avoid it. And you have to charge it to 100% so the Maximum Capacity can calibrate itself.
Have a look at the following:
👉 When to charge your iPhone or iPad - Apple Community (Must Read!)
👉 Maximizing the battery performance and lifespan of your iPhone.
👉 iPhone Battery and Performance.
👉 About the battery usage on your iPhone
If the Battery Health continues to decline at this rapid speed, have the device checked out at a Genius Bar or Apple Authorized Service Provider (AASP):
Have a great day!