Fun watch. I fish on Puget's Sound in Washington State, out of Anacortes. Just completed the Dungeness crab for the Winter, and Summer season isn't until May, or June. Time to stack, repair crab gear, clean the boat, make ready for fin-fishing as we head towards Spring. We have spiny dogfish here, different than the species you have there, come usually as by catch, but I never pass up the opportunity to make some fish and chips 😋.
Dead sandeel was never much good when I set longlines years ago. You want really fresh or alive. Mackerel, scad and garfish used to do well for the turbot ray and large bass. I did prefer the shorter lines though. You could get them into smaller areas of sand and gravel in rocky areas.
Greetings from the west of Scotland! I would recommend that you pass the hook up through the chin , makes a more streamlined bait and gives a better chance of a big flattie, and if the launce are always that big, cut some in half ( saves bait) and again hook them so they are presented better. I noticed a couple of Pirate 21's as you came back into port, it's what I have, nice safe boat, but gets a liter or two of water under the deck now and then, know idea how it gets there!