Shetlands are good crossers, yes. They can lamb large lambs but carrying them will have an effect on their belly muscles over three or four pregnancies. One very respected breeder uses Shetland first time, then 3 crops commercial, then back to Shetland.
Yes you'll get good breeding ewes from crossing. The classic cross on Shetland is the Cheviot, much sought after for ewes. I have several Shetland x BFL sheep and they're excellent breeders - albeit maybe a bit prone to triplets :/. So in your case I'd certainly use the Cheviot x BFL now, as if they've had several large tup crops already this might be the last time you want to cross them.