It's just my opinion....but during the warmer months I believe the bait is more scattered therefore the crappie are too. This means less crappie will find your lights at night because they already have baitfish they can feed on elsewhere. Winter months is different as the bait bunches up more, therefore less scattered and the crappies follow the food.
You can most certainly catch fish longlining during the hot months but it takes more attention to detail on the graph finding the fish. IMO not a good idea to just drop jigs deep and start pulling unless youre seeing plenty of fish deep on the graph in the area you want to fish.
My last two trips was early July and on the weekend. Plenty of boats both days. Since I was in a "fishing location" and not out on the main lake most boats I saw were fishermen though. One was an evening trip and I found fish on a 15'-20' flat and had to use double 1/16ths to get down to where the biting fish were at. I didn't try getting all the way to the bottom where most of the arches on the graph were at, rather stayed a few feet above them hoping the "active" fish would chase and bite. Caught 28 that Saturday afternoon in two hours of fishing. The other trip was early the next morning (same location) and had to remove all weights and one jig per line because the fish were shallower and I was pulling underneath them. Don't remember the exact count but was something similar to the previous trip.
All I'm getting at is.....use and trust your electronics and if you aint seeing anything then move on. It's not worth fishing for 'em if they aint there. Keep your jigs above the fish and remember that everything you see aint active. If youre marking fish and they aint biting then move to another place and start looking again.
If you do your part, once you find active fish you can and will catch 'em. And during this hot weather be prepared to go shallow in the water column when needed but always watch the graph because as the sun gets higher usually the fish go deeper. Good luck.