hi kv,
I think I follow,
Cheaper transformer will give unexpected voltage results when 'off load' compared to the expected voltage when fully 'loaded'.
This 'poor' regulation is due to cheap transformer construction.
If the 'on load' voltage is too high for the 30V relay, add a suitable value resistor in series with the relay coil
[ dont forget to consider the resistor wattage]
If the 'off load' voltage is too high for the smoothing cap, add a load resistor across the cap.
As you are running the application from the mains the loss in the resistors will not be a problem.