I had to use a full Windows install (You can install W10 and not register it and it remains fully functional).
Before that, I did mess a lot with Quickgui, my preferred KVM/QEMU solution which will setup W10 or W10 w/o registration, and in the end it may have worked: you do have to set it to auto-add new USB devices since this wonky MediaTek tools do some strange things booting the phone, etc. to get in “Meta Mode” for them to work, and if the phone mounts in Linux first then it will get taken over there.
BUT, also you need to flash LunarOS first as I note, and this could have been why my virtual Windows wasn’t working right, but after I got “real W10” installed again (yuk!) on spare hardware I just stuck to that install, and did succeed in the end. If I had spare time I could re-run the whole process under Quickgui, but it won’t happen for a bit I suspect 
BTW sorry my link above for the IMEI flashing guide wasn’t as clear as the later post I made on using SP Flash Tools to flash your IMEI. Maybe with that guide you can do it.
LAST note: Is AT&T also not an option for you? We have reports that AT&T network with US Mobile is working fine on the brax3.