[OnePlus 7 Pro Questions & Answers] Hutch MD:
Alright, so I was in the same boat as you, I bought a T-Mobile 7 Pro model earlier in the week; and nicely asked T-Mobile support through the App a few times to help me SIM-Unlock it so I could do my bootloader unlock and linked the Oneplus Support website and the Unlock page.

I ran into many issues so I'll help you out as I just finished my 6 day journey of making it functional.... Along the way, I lost cell service, I lost all my sensors.... So I'd recommend taking a TWRP backup everytime you're able to successfully boot into the device; Note: my goal was Havoc, so I ended up in a different place than where you're trying to go. I don't know what exact problems you'll run into, but I'm letting you know what I think makes sense from my previous experience over the week.

Your scripted-out plan:

0. Sure, test it, but you'll end up automatically wiping it later when you unlock the bootloader.

1. Sure, install the drivers.

2. You probably don't need to flash stock T-Mobile firmware as it's already running it.

3. You probably don't need to update it.

4. Yes, Flash it to international.

5. You probably don't need to update it. However I'd recommend to unlock the bootloader here;

5a. (The Damned T-Mobile model makes flashing TWRP a bit difficult) I'd recommend taking a note on what Slot is currently active (A or B).

Use "fastboot getvar current-slot" to find out; Then I'd recommend switching to the other slot ( If you're on Slot A, switch to Slot B, vice versa) to flash TWRP first (this overrides your boot image on your active slot, but is the only way to boot TWRP on the T-Mobile variant) and then flash the TWRP installer (this will flash TWRP to both Slots).

5b. After using the TWRP installer from your 2nd Slot, Use TWRP to swap back to the 1st Slot. Now make a complete full TWRP backup with all the boxes checked. Then, copy that backup to your computer so that it won't get wiped in the next few steps. (Do not skip this. If you did this correctly, You will have a place to come back to just in case you **** up your sensors, and cell service like I did.).

5c. Doublecheck if you can actually boot to your original Slot and if everything's functioning. Check if you can boot to your other Slot and see if everything's functioning. (This is where you super-need to make sure you have a safe place to come back to).

6. Sure, you can try flashing the latest Open Beta, but you might have to install TWRP again back in step 5b.

7. You might or might not have to unlock the bootloader, depending on what method you use to flash the Open Beta, your bootloader status might save; Sure, flash Magisk using TWRP.

8. Sure, flash xXx NoLimits using Magisk Manager.

9. Why undo your work and flash the Open Beta again? Don't do this.

With that you should be done.

If at any point you had an issue, I'd recommend using the TWRP backup you created earlier in 5b.

If you super-super have an issue, you'll have to use MSM to go back to stock International firmware.

In my case.... I had super-super issues and I had no TWRP backups.... and I had to use MSM to go back all the way back to T-Mobile stock firmware and start over because I had no service, and no sensors (proximity, gyroscope, accelorometer, etc).

But in the end, I think I have everything functioning the way I want to on HavocOS.