This is my take on the problem. Make the PWM signal into a 12V signal, then smooth it, finally drive the SSR unit with an emitter follower.
The diode in the base takes 0.7V off and the emitter base voltage another 0.7V, giving you just over 10V as the maximum. This will be fine, and you will get finer control than just using a three bit A/D.
Prop SSR.pdf (21.9 KB)