That's great
Now just to clarify
Chrome cast etc..is NOT USING WI-FI HOT SPOT
casting is a one way wireless connection, video stream to tv from your phone or tablet etc
Hot spot is a two way internet connection
In casting, your tv is an auxiliary second display, Not an internet connected device making and sending data
What you are doing is a great way to watch movies without burning data
These cellular video streaming options have only been around for about a year and keep changing with various promotions, hang on to your plan
I used to cable connect my tablet or phone to my tv to watch Amazon video and Netflix