Unfortunately zwift does not have power curves for the other resistance levels.
One option would be to get a power meter (wheel or pedals are the easier options) that way you can use all resistance levels and have true power indoor and outside.
Without a power meter the only thing you can really do is to go off feel. Do you know what for example 200 W truly feels like ?
If Zwift is reporting 200 W and you feel it’s too easy then increase the resistance and vice versa.
With the power meter, will the speed work accurately? For example if I increase the resistance I will go slower, will the Zwift adjust the speed in the app if it has power data from power meter?
What your curent setup does is it transmit the wheel speed to Zwift then Zwift convert that to a power number using a speed power curve then it use that Power number and some factors to calculate your wheel speed.
With only a Power meter you send zwift real power data and Zwift will calculate your zwift speed from that.