Call them ‘elite Infantry’ if Waffen bothers you.
These would be 2-2-2-4 units and limit of 6, with one build per turn. ( If you lose all six, it takes six more turns to get the build maxed)
In combat, they can be boosted by artillery 3-2 unit.
The faster speed is because the SS had their own train transport services.
I would rather the Waffen be represented by the tiger tank mold from 1941.
That would be a 4-4-2-8 unit ( limit six and one build per turn) you also can choose between building from scratch or convert one tank by paying 2 IPC.
The Stalin tanks could form “shock armies” ( same stats and rules as above)