The way I installed the update without any problems, coming from the 20200120 build:
-Reboot to recovery
-Make full backup in case anything goes wrong
-Flash newest firmware (20.1.21: https://xiaomifirmwareupdater.com/firmware/pyxis/ )
I'm pretty sure you are not supposed to reflash gapps when dirty flashing updated ROM (never done it on any custom ROM on any device) but maybe this ROM is different..
Yep, I know. Only used custom Google apps by gringo80..
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Have you flashed fstab? If you flash it, fingerprint will not work on any ROM.
The tip was added in the OP (that's what i'm doing, i just forgot to mention that :D)
Ok, so just to clarify:
Users followed EXACTLY your steps in OP.
Majority of them had problems with Play services/Play Store.