Fair enough obbn, just going with the most frequent issue I saw. It is true that most of the plans do have throttling (which isn’t called that, but I’m more than willing to call it that) which is generally for the top 2% of all users in an area, after which, yeah, you saw slower speeds. And having seen the bandwidth maps for the areas in questions, it did make sense from the Techs POV, even if it made customers grumpy. But if obbn tells me the current plan, I can double check if it’s one of the exceptions.
For @BigT’s point, I did actually see T-Mobile raise that 2% based on the increased availability and usage over the few years I was there. It was originally at or about 28G per month, but a year later it was over 30G. Currently, it’s over 50G per their website. In general this issue and soft cap is the same across all the unlimited plans, so it’s not normally used to suggest and upgrade - those come in the form of ‘the current promotion for 1/2 off or BOGO phones are only available to the most current plan’. 
For that matter, it’s actually more complicated - during times of peak use, everyone is going to see slower speeds, and those that are in this bracket are going to crawl. But if the usage isn’t high, the speed for the top 2% isn’t restricted at all. However most areas, even with 5g, are running closer to capacity all the time than anyone wants to admit.
One other piece of good news is that (in general) those top users are reassessed at the beginning of each new bill cycle, so you have to hit that bracket again during the month before it’s reapplied. Please do note, this was all from 4-5 years ago, so this may have changed somewhat.
If you’re pushing the 50G (you did say 30+ per month), that may be what you’re approaching. Other hidden ‘unlimited’ issues that came up (that I don’t expect apply here at all) is that if you’re in a roaming area, the owner of said bandwidth often severely restricts the amount of data used while roaming (in some areas to 200 Meg!!!), coverage on Cruise ships is billed by the cruise ship choices which is insanely pricey, and of course international has it’s own limits (better than cruise ships though!).
Since you’re in an area where there aren’t any other good options, I will avoid well meaning advice like ‘download over wifi’ and the like, which would be insulting. I would consider checking if the new T-Mobile Cell based internet service is in your area though - it was designed for ‘home’ levels of internet use (hundreds of G per month) for around $50 per month, but don’t know if that additional cost would be worthwhile for the pleasure of watching on a full sized screen.
The last two things I would have mentioned if I was still a tech would be 1) check your streaming preferences, it’s normally at standard, but if you’re doing anything in higher def, it hits that 50G much faster, and that may be a good idea to limit going forward. 2) in terms of streaming music, if any of it is stuff you listen to frequently (such as Amazon or Spotify playlists) it’s often easier to download it once and play from local storage (if you have the storage available).
Lastly, as a former T-Mobile employee (as distinct from a tech) I would mention that they tend to be very good at taking care of long time customers . . . who aren’t trouble. A few times, spread apart, they’ll bend or break the rules for customer satisfaction. But if it is happening every other month, they’ll give you that advice above and make no further adjustments. Which was already a LOT better than the experience I had had prior with other carriers before working with them.