Hi
@Mike2798 and warm welcome
👋I'm glad you are happy with s21 ultra, I still have my old s21 ultra and till this day it's still great device.
Now, the device will receive last major update next year, which will be ONE UI 7, and after that will receive security updates till 2026.
In terms of anti-virus you can install it, but it won't be as good as like on your PC as phone security updates is for a reason, as it designed to solve new security issues that might have occurred, or new bug that might appeared so installing anti-virus won't do much.
Best thing to do would be upgrade after device is no longer supported.