Welcome to ModelEngineMaker !If you have problems registering or logging in, please use the contact menu option to request assistance.
I wimped out and bought the casting - and finishing those nearly killed me!
Do you have or could you borrow a second torch to add more heat on the opposite side you are working? That can help when silver soldering deep or heavy assys. Just food for thought. Even an additional Spitfire type air/propane mixer tube plumbing torch can help.
What I would of first done was to solder the inter spokes since there is less mass there and the heat would not of transfer to the outer rim very much. After completing the inner then I would concentrate on the outer rim with heat specifically in spots like two spokes at a time then move to the next two etc... just my two cents ..