On second look, the "international" version is Europe and North America, so the one you have is the "international" version. Maybe with TMobile bloat, but it should still be the same phone. The problem may be that because TMobile has made modifications, you only get updates from them, not from OnePlus, (According to
https://forums.androidcentral.com/o...-oneplus-6-6t-may-android-security-patch.html, the E/NA version is up to the 1 May 2019 update. Changing ROMs won't change the IMEI, so the update situation won't change.
The official 9.0.13 (April) is at
https://forums.androidcentral.com/e...softwareupgrade/details?code=9&token=GypVI6WT. OnePlus gives instructions for using that to update.
Oh, ignore this - you did all the research.
As far as the Q beta, there are still a lot of apps that won't run on it. I was on it for a few versions, but after the 3rd one, I went back to Pie. Just too many apps that I use that weren't working right yet. (And I don't blame the developers - rewrite during beta, something gets changed and you have to rewrite again. Once Q has been out officially, I'll start asking for fixes. And since the phone is rooted, and I do the updates myself [it's a Pixel, so both the update and the full factory ROM are published every month], I can decide when enough apps have been fixed so that I can update. [The update won't install, even if it's forced OTA, because of the rooting.]) But OP seems to be doing the same as Google - publishing the updates. So as long as you have an "official TMobile" ROM to flash, there's no reason to not convert. I'd save an apk of their text app, so I could install it after converting, but you can always flash back as long as the phone can talk to the PC. (And if what I suspect is true, even if it loses the ability to see the PC, and you're stuck in the bootloader [which is basically hard-bricked], just holding the power button down until the phone vibrates will fix that.)
And check
https://www.xda-developers.com/download-stable-pixel-3-google-camera-port-oneplus-6/ for a OnePlus 6 port of the Pixel 3 camera. No Visual Core processing, but the rest of it's still pretty nice.