The 5Gb Canada/Mexico high speed data IS separate from the other international 5Gb feature. I recently used it in Mexico and then 3 other countries and it registered separately. The 256k data does kick in when the HS is depleted. It is slow, but useful for texting, email, mapping, navigation, and text based browsing on sites like Flyertalk.

I have 4 free lines and use them in iPads and they receive the 5Gb data services also. I do not know what causes this, and on Reddit-T-Mobile, no one else seems to have a finite answer either. It is NOT prohibited by any of the Terms Of Service from T-Mobile. This issue has been argued probably once a month on Reddit for years. Suffice it to say, try and see what happens if you have a spare line sim to add to an iPad. Be sure your 911 address is current, and you have made calls with that line. Worst case if you have a dual sim phone, each line will get the 5Gb, so you can have two lines @5Gb each for hotspotting to your iPad.

Edited to add: I have been using voice sims in my iPads for over 5 years now, and have not heard a peep from T-Mobile about it. I always keep an unlimited free line in an iPad Mini 5 for use around town without any issues.

While in Canada and Mexico, calls to US numbers remain free, even if they are also roaming.

Netflix is about $5 extra for 4K I think, but I don't pay it and it looks great on my sets.

Can you just upgrade one line for trips, or add a feature instead of paying more always? Seems like it might not be worth it if you have more than a line or two. I have 12, but I am on an older ONE plan with a bunch of free add-ons like ONE Plus Promo, which is discontinued.