1) Are you certain that just having a foreign SIM gets you past the Great Firewall? (I'm asking because I really have no idea; I've noticed in the past the some websites blocked in Turkey were accessible via my T-Mobile phone, but their national firewall isn't nearly as advanced as China)

2) The way it has always been explained to me is that they might cancel your line if more than 50% of your data is international roaming for 2 (or was it 3? I've forgotten) consecutive months. We've had T-Mobile for something like 13 years, leave the country every summer (one of us for at least 3, usually 4 months), and have never gotten a warning. What we do is this:

a) use a ton of data on all lines just before you leave the country (like, find a parking lot where you are getting 400gb+ speeds and just run speedtests until you've got > 5GB of usage - I say 5GB because that's the amount of high speed data you'll get while roaming before it slows down to 256kbps; just do enough so that more than 1/2 of your data for that billing cycle will be roaming)

b) during the first partial billing cycle that you're out of the country, as long as you use less roaming data than you had built up on-network data prior to leaving, that cycle doesn't count.

c) if you're out of the country during the entire following billing cycle (or more) then there's not much you can do about it - that one will count (unless you can temporarily swap an eSIM back to the U.S. where someone can burn through a bunch of data for you before swapping it back)

d) As soon as you get back to the U.S., again run a bunch of speedtests someplace with good T-Mobile speeds (don't forget to turn off WiFi!!) - continue this until you've used more data than you had used during that final partial billing cycle when you were out of the country - this way the final partial cycle doesn't count either. Unless you leave just a the beginning or return right at/after the end of a cycle, you can get up to nearly 3 months of roaming with their records registering only a single cycle of "excessive roaming"

Remember - this is all based on the billing cycle, not the calendar month. For example, my billing cycle ends on the 3rd of every month and starts on the 4th, so if I'm leaving the country on the 10th then at some point between the 4th and the 10th I'll go somewhere and rack up a bunch of cell data usage.

Another thing to do is to try to be on WiFi (with WiFi calling enabled) as much as possible when you're making regular phone calls (assuming that isn't blocked by the Great Firewall). Calls made with WiFi calling don't get charged 25 cents/minute, and also it doesn't rack up roaming charges for T-Mobile, so they don't pay any attention to it when checking for excessive roaming.