It would be a HUGE upgrade if there was a smaller tire size to choose from than 20”. I’m currently using a spin bike with a 16” flywheel so my numbers are coming out faster than it really is which is causing me to get flagged on zwift. Not everyone can afford smart trainers and I LOVE zwifting but it’s becoming frustrating as I bike almost everyday and have been for years and I’m looking to get even faster.
A secondhand road bike (assuming you don’t have one) and a secondhand wheel-on smart trainer would give you a much better Zwift experience than you’ll get from a spin bike.
That does not work because Zwift does not know at what resistance level you are and then can’t have a power-speed profile.
Adding power pedals or crank is a option if it is compatible.