Unofficial twrp should be ok too and usually is patched earlier than official version.
I don't root my devices anymore, but in the past often Magisk or some modules caused those problems not booting up...
Seems you have to trial and error...
I don't have such problems with latest original twrp-recovery...sometimes the Gapps-backup-script is wonky and Gapps don't get flashed through update process, so I have to flash them manually afterwards, but device boots up.
Maybe switch to twrp-recovery...else: Did you root you...
Usually gcam is linked with google photos gallery, so if using third party galleries, this unfortunately is expected behavior. If I tap the preview button in px cam, screen shows gcam-ui with a black background.
First boot encrypts the storage, so you have to format data partition and confirm with 'yes' in order to boot the rom coming from unencrypted xiaomi.eu weekly. Just wiping each partition in twrp is not enough.
You can decrypt in TWRP in wipe section -> FORMAT DATA -> enter "yes"...whole storage will be wiped AND decrypted. Reboot TWRP!
Then transfer rom file and flash it. Arrow boots and storage is still decrypted.
Dev will tell us when Android 11 is ready for running encrypted again in future.
For firmware visit: https://xiaomifirmwareupdater.com/firmware/polaris and choose V220.127.116.11.QDGMIXM Global.
Custom Roms have their own vendor-partition, after wiping and formatting you can flash the rom in TWRP.
If Xiaomi.eu-Rom still is unencrypted, you'll have to FORMAT the data-partition in TWRP (not only wipe) in order to flash and boot LineageOS afterwards. FORMAT data (you have to confim it with 'yes' in TWRP) wipes the whole internal storage of your device -> so backup your files (photos, music...