[OnePlus 9 Pro] Appreciative:
You can unlock your bootloader and root while on the stock Tmo firmware and unlock the sim whenever. If you're planning to stay on stock tmo and worried about losing data, you only lose data when you un/lock the bootloader, root is fine. I can first hand tell you that I did both the tmo and EU bootloader unlock bypasses while I was sim locked. Then I flashed back to stock Tmo, unlocked my bootloader, rooted, got the unlock approved and monitored the https packet exchange for the unlock process. I was trying to see what could be figured out to create a method of self-sim-unlocking. But I didn't figure enough out.

It would be possible to unlock bootloader, root and unlock the sim on the stock Tmo firmware later, for sure, because that's how I did it. But what I can't say for sure is if updates from tmo-oos11->12->13 relock the bootloader as I didn't update to 12/13. I know in previous Tmo phones, you can't easily do the unroot/reroot because the tmo firmware automatically restarts the phone after (temp unrooting and) taking the ota. I can't remember for sure if that ota relocked the bootloader or if it was just ota. Ghost has updated from tmo11 up so he should be able to advise for sure if taking the ota relocks the bootloader or not. I wanna say it doesn't but I'm not sure enough.

There was no way for me to sim-unlock the device while I was on EU. I did look into doing this idea but was unable to try it:

Use tmo->EU msm tool

Unlock bootloader and root

Modify the modem configuration files with qpst to add carriers, bands and access points. I don't know if that can work. I was told it can't work, I was also told it DID work on other devices in the past but found nobody who had tried it with the 9P. I didn't have a second sim to test anything with.

I took a backup of my modemst1/2 while sim locked and again after unlocking. That's a good plan for anyone but especially if you're going to start playing with qpst or modem files at all. Having both, I should be able to flash modemst1 and have a sim lock again, then unlock again. I was never able to find any unlock.apk from Tmo that worked on the EU rom to actually do anything. Even when I had the greenlight from tmo, I had to flash back to stock Tmo firmware and use the built in unlocker.

I'm not sure if I mentioned it already but i did need to be on Tmo network for a few days. When I first tried, I had no sim inserted at all, they never got the 'first packet' to register my device on the network. From my reading of how the sim lock/unlock works, after a stock tmo msm flash or during first phone set up, the phone sends in a registration packet that has a unique identifier (looks like our serial number but it's not) along with our IMEI. It seemed like it took 2-3 days of being on the network until support chat changed the story from "we don't have that IMEI in our system" to "we have that IMEI but you need to contact OnePlus, our regular system does not unlock 9900xx IMEIs". I believe you've already been on the network for days but I'm mentioning this for future googlers also.

Btw, I never told Tmo I needed my phone unlocked for another network or leaving the country etc. I told them I'm an Android developer and I need the sim unlocked so I can unlock the bootloader. (The agents are pretty unaware in general but they definitely don't know that 'requirement' for bootloader unlocking can be bypassed easily in our devices) I read too many stories of people saying they needed it sim-unlocked for another carrier and getting the run around.

I was tagging @TmoHelp and a couple other T-Mobile accounts on twitter, every day until it got solved.

Since I ramble on, back to your first question, it is for sure possible to unlock bootloader, root, and then sim-unlock while rooted with ubl. I guess the next question we need a for-sure answer to is if you can then take the OTA to 12/13 on the tmo firmware without it relocking the bootloader and losing everything (you can on EU but you can't sim unlock from there so that doesn't help). @TheGhost1951, do you know for sure if he can OTA tmo-oos11->12->13 with unlocked bootloader and it stays unlocked and fine without losing data?