Thank you for the post above.
I was unable to flash the TWRP linked above, but using the latest unofficial TWRP image worked: https://forum.xda-developers.com/Mi-9/development/recovery-unofficial-twrp-xiaomi-mi-9-t3905825
For me at the time of writing that is version twrp-3.3.1-42-cepheus-mauronofrio
However I fail to install LineageOS (lineage-17.0-20190922-UNOFFICIAL-cepheus.zip).
I’ve noticed that the zip contains patches for the system image. What version of Mi OS is required for the zip above to work? I have the latest global version installed, version 10.2.30.0 20190806 directly from Xioami’s website.
My device is unlocked.
I’ll be happy to provide any help I’m able to provide by sending you debugging data.