Why a slow cooker. Bigos catches on the bottom of a pot. On the hob you are tied to the kitchen, stirring every half hour for four or five hours, and if it catches even once that burnt taste runs through the whole batch. A slow cooker has no hot spot, so there is nothing to scorch and nothing to stir. You can still make it in a heavy pot on the hob or in a low oven if you prefer, but keep the heat as low as it will go and stir regularly.
On the liquid. The pot should not be swimming at the start. Cabbage gives up a lot of water and a slow cooker loses none of it, so bigos loosens as it cooks rather than tightening. If it looks tight after a couple of hours, top it up with a little hot water. If it looks loose at the end, take the lid off for the last half hour.
Season at the end. Sauerkraut and kielbasa vary enormously in salt, and the pot changes a great deal over five hours. Taste it before serving and adjust then, with salt, pepper or a little more honey if it needs rounding out.
To serve. Sourdough bread or boiled potatoes.