Can I set my mac to stop charging while I'm using it & it's plugged in?

Under System Settings > Battery, click the 'i' at the far right of Battery Health and make sure that Optimized Charging is turned ON. That will pause charging at 80% full when you are running plugged in most of the time.

Your battery is not going to last 8+ years, and by some miracle it does, your phone will be obsolete by then.

Just use it. Getting the battery replaced isn't that expensive if it needs it in a few years.