Here's my proposed schematic.
View attachment 292441

Is this right?

 

Is this right?

Yes, if the input and output grounds are connected together internally on the converter board.

 

Yes, if the input and output grounds are connected together internally on the converter board.

If they are not connected together on the converter board should i connect a wire from the input gound to the cathode of the thyristor?

 

If they are not connected together on the converter board should i connect a wire from the input gound to the cathode of the thyristor?

Yes, or any place between the input ground and the HV ground.

 

I don't see the point of triggering the SCR from the 12 volt side if the module is isolated, I would just trigger the SCR from the 200 volt side and keep the isolation. (with the proper resistor)

Also, the idea to place the power switch on the 12 volt side is a good one. (from post #19)

Edit:

After thinking about it I guess if the OP wants to use the entire range of the booster it would not be possible to find a single resistor to do the job. But from 200 to 390 would be possible.

 

I don't see the point of triggering the SCR from the 12 volt side if the module is isolated, I would just trigger the SCR from the 200 volt side and keep the isolation. (with the proper resistor)

True if it is isolated, but I see no mention of that from the TS.

 

True, I was just speculating...also see my edit above.