Ok here is my fix for the thermoster replacements.
I used 2 old thermosters and gutted them. Then I got a 90° hydraulic fitting and cut off the threads. I also got 2 12mx1 nuts. And welded the pieces together. The gutted thermosters act as plugs and the nuts are to put new thermosters in, I also added some heat transfer paste in the fittings. Lastly I plan on heatshrinking the fittings and adding some pipe insulation over that. So now when I change out the thermosters I don't need to vac and refill the system..