That's my main concern, bored dog. Im working with 350(something) ewes so by no means is everyday a busy sheep day. But are times when I feel it's be useful. And (selfish) I just really want that bond of working alongside a dog.
You've got plenty of sheep, you want the bond of working with a dog. Get one, train it - there's nothing like it.

:hugdog: With that many sheep you can sort out a few youngsters to train the dog on, and to have a practise session with from time to time to keep both your hands (and paws) in. Once the dog is trained and used to the routine, it will, as others have said, settle into it and be happy to do other stuff with you when there's no sheep work to do.
Dot loves chasing swallows, riding on the quad bike, playing with a toy, herding chickens and ducks, or just pottering about alongside me whatever I am doing. She wasn't always like that and when younger I did have to keep a close eye or she'd be off finding sheep if I was busy with something else. But you'll fathom what works and what doesn't with the dog that you get.
Collies need more down time to think about things than other breeds. When you're training you will see very clearly that the dog has processed whatever happened last time and thought about what it might do in a similar situation next time. Provided they get plenty of exercise - of body and mind - they're very happy to spend hours in their pen / crate / stable / kennel or whatever is 'their' place, mainly sleeping and thinking. The odd bone doesn't go amiss
