I started cycling a few months ago and found a cheap used trainer that was a great introduction to the sport. I now want to upgrade to a smart trainer that can measure watts to keep track of performance.
I don’t want to spend more than $500 USD and I would like it to be compatible with TR and Zwift. Any suggestions would be greatly appreciated!
The Elite Zumo might fall in that price range the Elite Suito is just over but we are ripped off in the UK so it may fall into it over the pond 
If you want “smart” meaning controlled resistance for workouts (ERG mode in TrainerRoad) and simulation of hills (like SIM mode in Zwift), you have a few options around that price.
Kind of the defacto standard for this price point:
Just a bit over at $550 USD, and not my favorite if I’m honest:
Also over the price a tad at $580 USD:
https://www.elite-it.com/en/products/home-trainers/interactive-trainers/tuo
Just for reference, you can use a “standard” trainer and a speed sensor for VirtualPower in TrainerRoad and similar functions in Zwift, to estimate power. You won’t get feedback and automatic resistance changes from a true smart trainer, but know that a smart trainer is not a requirement for either app.
This means the trainer you have right now may be a fine starting point without the need for more investment (assuming you have a wheel speed sensor), as long as you are OK without the controlled trainer function. Plenty of people still train with these simple trainers and get great results with them.