Some good ideas and tips here.  One thing I do differently than the recipe states, is that I always fry my hamburger with just a bit of chopped onion, first and then mix all the rest of the stuff together before putting it into the cabbage leaves.  The idea of freezing a head of cabbage sounds kinda weird, but it works!  All you have to do is find room in your freezer for a whole head of cabbage, so I use one of the bigger freezers we keep in the basement, rather than taking up so much room in the freezer part of our fridge. It just requires a little running up and down.

https://www.thespruceeats.com/swedish-cabbage-rolls-slow-cooker-3055201

***  I have never made cabbage rolls in a slow cooker, I always use the oven.  In the summertime when it's too hot to use the oven, I use my roaster outside just as if it were an oven - - same difference!  The outcome is better with an oven than a crock pot only because you can take the foil off the top and brown them just a little bit.  Can't do that in a crock pot.