can you solder 2 copper plates together? and have non-corrosion + adequate thermal transfer? Milling is not an option for me
Issue there is warping with the heat involved on two such comparatively large surfaces... It could be done, but controlling the deformation would be tricky. You're moving from electrical soldering into mechanical (?) soldering there... then you've got maintaining the pressure of the two plates against each other while you solder... where the clamps would be, or vertical load weight, would affect the heat dispersion and thus the thermal quality of the bond...

S'doable, but guaranteeing performance of the end product becomes difficult.