How to cook frozen goose
Please remember that you are talking about poultry breasts, which are always the leanest part. So no extra long cooking to tenderize, you'd be just making them tough and dry.

I'd also recommend not putting them in the slow cooker, because the skin is rather fatty and you'll want to render it to get the skin  nice and crisp, let the excess run off and moisturize the meat below. In a slow cooker, most fat stays in the skin, making it rather rubbery (and IMHO quite nasty).

You should be absolutely fine if you either


sear them skinside down in a pan, then finish in the oven or
just pop them in the oven for two hours (rough estimate, depends on weight and oven temperature)


Of course you can freeze the leftovers.