I went through this process and some of the files failed to flash leaving me with a brick. After trying some things i was able to get it back up and running by running an oneplus 9 pro msm and then using TWRP was able to install a one plus 9 Global rom. Unfortunately my service is being crap now, even after flashing the modem in the OP, so i wanted to go back to stock tmobile but whenever i run the msm for the stock tmobile rom i get an unsupported target error.
Any ideas on how to get around this?
My service was crap too and reverting back to the T-Mo ROM didn't help at all.
Try downloading the Qualcomm drivers from the unbrick site and see if that helps. It solved my problems. I wasn't able to flash anything before I uninstalled and deleted the drivers I was using (it was a new laptop) and then installing those. I went right back to T-Mo from EU though and skipped the Pro MSM.
I tried the Pro ROM, though, and it wouldn't work for me.