S21 ultra need suggestions

Hello i need help i have s21 ultra original model was sm-g998u but (tmb) but i changed it sm-g998u1 (xaa) android 12 ui 4.1 im trying to downgrade a to android 11 ui 3.1 but it wont let me the reason is i want to rooted oem wont appear i tried lots of tutorials it wont so maybe downgrade it might work i don’t know i hope someone can help