Not really ansering your question, but a quick comment on the Zwartbles: We're similar, having started with them purely because Mrs Womble fell in love with one that chewed her sleeve at the Royal Highland Show
. There's a lot to be said for keeping what you like though. After all, it's you who has to go out and feed them all winter in the dark, rain and snow!
I've since found out that not all Z's are created equal. Some are very leggy 'Dutchy' types. Some are also much larger framed than others, so logically they would need more feeding. So rather than buy into a breed and then try to breed out your undesirable traits, perhaps you could get closer to what you want by being very selective about what you buy to begin with.
We've just bought a smaller framed Z gimmer, who looks pretty hardy to me. Only time will tell if she's as good or better than the showier ones we started off with! Of course if you don't want to show, there are plenty of mis-marked but otherwise great Z's about at reasonable prices. Or, if it's all about the stripes for you
, what about
Balwen Welsh Mountains?