I use Himalayan salt licks on a rope. Our local agric store doesn't stock them so I get them from Ebay - they weigh about a kilo each and cost more to send than to purchase so I get a few at a time, but then I have more sheep than you do. They take it as they want it, same with licky buckets, sheep know what they need and they are not all the same. One tip with the rope, keep it taut so they can't get their heads in, and take it away once the lick's gone, to prevent accidents.
For licky buckets, ours only get them in winter. Because they get willow, the bark has plenty of minerals etc under it so I don't feel they need supplements in summer, but judge it according to your conditions.
For condition scoring of course Shetlands being primitives should have a fairly low score, as they don't store fat on their backs. 3 sounds fine, not more than 3.5 though, but honestly I wouldn't worry too much. A handful of rattly feed is fine every now and then, you could even up it a bit if the snow lies for long.
Cross posted with Sally