You are right. I though about it but then figured that if I am buying a used trainer with some other equipment to add up to 100$, then I would rather wait a bit and buy a used powermeter. Some of those things can be had for low 200s on ebay and that would solve the problem entirely even when using an unsupported trainer. If zwift/ roadtrainer can get direct and accurate power data they should be able to portray my speed on hills/ flats similarly to others on supported trainers.
I was hoping for a magic solution in the meanwhile. Im using it with a setup as a random other trainer and so far roadtrainer gives me reasonable numbers I think. My FTP ended up around 150 when I did the test which is what it predicted it would be. Im relatively new to cycling.... average around 15-16 mph on a 1 mile park loop with hill and descent. The number I got sounded reasonable but I just don't really know for sure so was hoping for something more concrete than me just picking a random trainer/ resistance combo and hoping it was similar to what I had.