I've had T-Mobile for many years. Generally speaking, it's always the best cost/value out there.
If you are not using a hotspot, you will only get slower speeds if you are connected to a tower that is being heavily used. This is rare.
If you use up you data while using a hotspot that they can figure out, you will get much slower speeds after the 50g. Down to 3G...but maybe not as they are closing down 3G, right?
You can buy extra hotspot data, its not "real" expensive. The cost should be available to see on their website.
I have not check in several years if any of the old methods of hiding the fact that you are using a hotspot can still be hidden from the carrier.