I did once find a good and genuine retirement home for some of my older ewes, too young to kill, too old to sell for breeding. But that's once in 20 years

It's a shame you can't keep them separate from the younger ewes as that's the way to go. They can then get enough supplemented feeding up until the spring grass comes through. We have kept older ewes with an excellent condition score to a great age (remember Jezebel who was just short of 20, and had only started to lose condition in the last year?)
As WBF suggests, when they have some teeth but other gaps is when they struggle to eat enough, especially if one of the remaining teeth is wobbly. It's those wobbly ones which are pulled out and that does help with their eating. I wouldn't pull firm teeth as it would hurt like mad, and the gums would take a time to heal. Once all the front teeth are gone, and there are no problems with their molars, they can eat fairly normally, although they need grass slightly longer than younger sheep.
We have found that they might take a couple of summers to put back enough condition to be converted into burgers and sausages, after their last lambing. Primitives tend not to put on any condition at all over the winter, be they young or old, so it's all down to plentiful grazing in the summer, and support to keep them going over the winter. And of course, Digestive biscuits

Like you I hate marts and we don't sell any of our animals that way. For your older ewes, if you can get them fattened, then have them made into nice spicey sausages and sell them in that form - that way you know how they've died as well as how they've lived.