Adult sheep that are healthy don't need routine worming more than once a year - ewes that are lambing is the only routine dose, beacsue the stres oflambing lowers their resistance and the worms ramp up egg production. So the worming is really to protect the lambs rather than the adults. And some folk don't even subscribe to that often!! I've never thought combi wormer / flukicides are a good idea beacsue you'll be treating for fluke more often than you need to worm, so you'll be overworming.
You can give a wormer and a flukicide at the same time, but not mixed as you rightly say.
Lambs don't really start eating grass until about four weeks old; takes three weeks or so for worms to be a problem, so they shouldn't need wormed before 7 weeks of age. Like you, for nematodirus, I keep an eye on the local forecast.
We usually fluke three times a year - October with triclabendazole and January and lambing with closantel. If we have a very wet summer, I'd probably do the lambs at weaning, being mindful of withdrawal periods.
If you don't have many sheep, ask your vet about wormer. We only have a few now and our vet just gives us what we need. It's more expensive per dose but less expensive in total cost and, of course, far less wasteful and we can rotate wormer more easily.