Sorry for starting a new thread, but I struggled a lot finding the right steps, so this might help someone (or at least the future me).

Preparations

I was using MacOS something (should not matter).
I already had some old-ish version of platform-tools (adb, fastboot & co), the version should not matter, as long as it is from this decade. (See the note later about mke2fs on MacOS.)

Stuff downloaded:

• vendor.img (https://github.com/ubports-xiaomi-sdm660/artifacts/releases/download/v0.1/vendor.img; a newer one gave me checksum error)
• TWRP (https://eu.dl.twrp.me/jasmine_sprout/twrp-3.5.2_9-0-jasmine_sprout.img.html; I tried different versions, but this one worked fine; also tried the stock recovery, but it was complaining about the new build being older)
• Android 9 stock image (https://github.com/ubports-xiaomi-sdm660/artifacts/releases/download/v0.1/jasmine_sprout_stock_android9.zip; as I was starting from 10)
• installer (https://devices.ubuntu-touch.io/installer/dmg)

Steps

Unlocked bootloader by becoming "a developer", enabled USB debug and OEM unlock, then booted in Fastboot, then

./fastboot flashing unlock
./fastboot flashing unlock_critical

If if gets locked again (it dod for me somehow), just check status with ./fastboot oem device-info and unlock what is locked...

Booted in TWRP from Fastboot

./fastboot boot ~/Downloads/twrp-3.5.2_9-0-jasmine_sprout.img

TWRP asks about mounting stuff read-only, I just went with "no".
Not sure about wiping any cache/data, as my phone was empty, I just wiped everything if I got stuck, might not be necessary.
The phone has 2 "slots" (A & B). I had to install Android 9 on both. (You can check the active slot with ./fastboot getvar current-slot, and select a slot with ./fastboot --set-active=<a|b>, but as TWRP installs the new image on the passive slot and makes it active after, I just repeated the steps again to install Android 9 on the other slot.)
Then in the TWRP menu I went with "Advanced">"ADB Sideload", then

./adb sideload ./jasmine_sprout_stock_android9.zip

Wait.
Reboot into Fastboot, then TWRP.
Repeat.
Thanks to Apple's "security" at this point I had to do a ./fastboot -w to run mke2fs and get it blocked by MacOS, then allow it from "System Preferences">"Security and Privacy", otherwise the UBports installer might fail.

Finally on stock Android 9, I started the UBports installer, followed the steps exactly, and got UBports installed.

Misc notes

• Neither "Mi Unlock Tool", nor "rooting" was necessary.
• For some reason after a point the device was detected as jasmine, I did not care, just selected "Xiaomi Mi A2" from the list.
• I tried to use vendor.img from the latest stable artifacts (https://github.com/ubports-xiaomi-sdm660/artifacts/releases/tag/20211023), it gave me a checksum error, but as I was retrying a few times after using the vendor.img the installer says, idk if it would work or not.
• If the phone bootloops/etc. don't panic, just go back to Fastboot and repeat.
• At one point my PC did not detect the phone in Fastboot, but doing a random combination of "phone restart", "USB unplug/plug" and factory reset using the stock recovery fixed it, idk...
• Getting into the menu of the stock recovery is tricky, I do not have any good advice. My technique is to press power+vol up/down furiously until the menu appears