How to recover glitching Galaxy S9?
bootloader level is increased in 1 digit
1 2 3 4 5 6 7 8 9 A B C D
G960FXXSBETH1 = B
G960FXXUCFTJ1 = C
you can download rom for SM-G960F from updato, samfirm, frija or samloader. try lowest bootloader first, as you already stated downgrade is impossible (and higher version usually comes with higher protection against rooting). you can check bootloader version from download mode screen