It's a great phone, mine has been a super replacement for my Huawei P40pro+.
I'm on current beta now but I came from the original Chinese release through fastboot before a twrp was released.
Best bet before you start unless you are really not happy, is to do a back up by twrp and take it off phone as a backup.
And as you are starting from scratch again might be worth updating to latest twrp for mi 10 ultra. I have a link from my mega page here...
https://mega.nz/file/TUh0mLAZ#0jYRCIeAiCJEMW1eIvWzPSWG2MuKJMG5V-wgxtWT80M
Ignore the Chinese bat file and install it through fastboot. I take it from your comments that you are ok with PC and fastboot/adb but if not, here is a link for my mi10ultra ROM installation, it has a twrp section but you will need to rename the new one to match. Guide is here...
https://mega.nz/file/6UAhXCxZ#Np2KwhMnl8obmOvp4b-2R9ajo0zrtz3vwefy-Ya3PLs
Then download the latest beta ROM from the download page...
https://xiaomi.eu/community/threads/21-1-13-14.59414/
and get it on your PC or OTG usb stick, not your phone as it will need to be formatted as your changing ROM branches.
Install TWRP, then boot straight to TWRP, not system. Then FORMAT data from TWRPs menu. Mount PC or OTG usb stick with the ROM on, using TWRP, install ROM, then once done, FORMAT data again, then reboot system. Obviously you would normally only wipe dalvik and cache on any same branch update but as the EU ROM and Chinese may have partition differences between them, it's best to format before and after on this install only. Then wait, it takes ages but as long as you see the 5g and MIUI logo, it will be ok.
I know you have done this before but please, follow, this ish. Reinstall TWRP and flash ROM as if it's a stock Chinese but with the added format before flash and after. See how this goes. If you still get errors while in twrp, you can press vol down and power to take a screenshot of the error and let me know.
Worst case, use twrp and put your back up back on or go to
http://mifirm net and put a more recent fastboot ROM back on and try again but let me know how you get on. Good luck.