Hi Alex - welcome (almost) to the world of Zwifting. If you dont have a Zwift supported trainer (you dont appear to), then you need to transmit power or speed to Zwift in some manner. Easiest way is normally a speed sensor but as you mentioned, you cant use a speed sensor so you need a power meter. Doesn’t look as though a crank option is viable for you so you would need to go down the pedal path.
A second-hand smart trainer is likely a better choice for you though (cost and experience wise).