Why can’t you just
adb sideload the latest OTA zip?
It is no different than manually updating across versions on stock OS.
Fastboot files should only be used for initial installs, and as you mention can only be flashed when unlocked.
OTA files can be used from system updater apps and the recovery, and work when locked as long as signing keys match.
I have multiple bootloader locked devices that have been in-place upgraded from 10 to 11 to 12 in real-world use.
Here is a bootloader locked and verified boot enforcing sailfish that went from 10 to 11.
LineageOS only added 11 support in February of 2022: What did the marlin say to the sailfish? · LineageOS/hudson@4b0dfba · GitHub
Yet many application were installed November of 2020.