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I expect I only use about 40% of the cabbages I plant and then only 30% of each one by the time it is cleaned. I do plant many more than I need so as long as I have enough to pick one or two a week I get by. We use a lot of kale for our green hit but you can't beat a nice crisp savoy. I don't know if it is an old wives tale but really firming them into the ground when planting is supposed to firm them up so the middle is nice an tight and bug proof. It certainly works with sprouts and stops them blowing. I dread to think what is in those shop bought cabbages that stops all the bugs and earywigs.
Kale may well be the answer. There are perennial kales like Daubenton that just look after themselves for years. Occasionally you pick off a stem and root it. But the plants go for at least 4 years and some have them for longer. By that time the cuttings will be a substantial size and can be picked all winter. But they need their space.
I find Cavalo Nero very bitter, but like you I love the purple midribs of Taunton Deane. They are so fat and delicious this time of year with a little bit of leaf attached as the leaves are not at their most tender. In spring the greens are the best. Coleslaw sounds a very good idea. This year I had cabbage whites caterpillars on my early sprouting broccoli but the Taunton Deane right next to it, escaped. For perennial broccoli you can go two ways Vinlander. Either grow perennial cauli Nine Star or Portuguese kale, which is also a perennial aka Tronchuda.
That may well be where I went wrong. While there is still plenty of salad, I tend not to go for kale. Late in autumn it may have been too late which is why mine were bitter.