It's just an anecdote, but from my one-time experience the 90 days restriction isn't a hard restriction with TMobile. Service worked fine for 5 months for a family member abroad (in a single country) with no issues or messaging from TMobile about an overstay - although this was on a family plan with other phones in the US for the majority of that time.
Google Support tells me it's a Google company policy, not a U.S. policy. And for comparison, Verizon works fine, just by paying a bit extra for extended roaming.
people in the rick steves link say you just tell tmo when you get back in the us to reset it/turn it on for the us. This should not have unclear limits of course.
The issue with having an explicit threshold is super heavy users will use right up to the limit and recommend it to other super heavy users. This MVNO then gets a disproportionate percentage of unprofitable roaming costs. The solution is for them to pass through the costs so there’s no dancing around the costs by suddenly cutting you off.