Alright, data from having a cavline for two weeks:
Luna FE, a competitive FEHtuber, says that “the goal of a cavline is to create as much turn 1 pressure so that the enemy is forced to tank undesirably”. Cavlines consist of strong independent cavs who try to cover as many of the units they’ll encounter to make it difficult for the attacker to keep all their units safe. Cavline gets way easier to tackle if you don’t have buildings, so serious players who do that will probably win straight out unless you’re playing on Springwaters with WoM shenanigans.
Traditional cavlines try to spread their influence out to as many tiles as possible, not leaving room for space. The desert map is ideally used for people who don’t have two mythics (AKA, people like me), and those who have two mythics (or… two Peony’s like what @Marbels did with his cavline a few weeks back) I’ve seen going on Spring Breeze and Springwaters. Traditional ones have covered 4-5 lanes, I’m not sure about how well in comparison going less does, but I’m not too big of a fan of that idea because less means it’s way easier to get the cavs baited for free.
Structures are very important on cavlines. Panic manors, shrines, and schools I’ve seen to make the most impact on a cavline. I concluded that infantry school is the most important to consider for dark cavlines, because of how Mila is thicc and that most of the counters you’ll find are stuck on infantry units (NCD generally). Infantry units are also the most popular carries to use from their insane skill availability.
Your primary goal when building a cavline is to make sure that everyone covers the mythics fairly well. While Mila has given archers a harder time from her def boosts, I’d still argue that not giving Eir or Peony a place to breathe is makes them decent enough and that the def boosts just makes the defensive stat boosts more balanced. On Dark, it’s pretty difficult to make good use of healers when it becomes an easy place to put Mila or Eir. I find that any time a mythic can step onto danger zone is a consistent loss of pressure.
Speaking of which, Astra is definitely better for cavlines than dark is. It’s easier to bully Naga and Altina, there’s no res boosts, Mirabilis allowed some really dirty ruse rally traps, Duma’s Upheaval damage becomes relevant, and Thrasir can be a very menacing WoM punisher if the cavs survive. On Dark, healers suck more, the mythics are actually useful (and being ranged can help with turn 2 cleanup), Eir with Temari and Mila with Sabotage Atk can be sure to reduce a lot of pressure, and having a free Peony on the mythic slot means turn 2 cleanup is less punishing from how out of position players can get from turn 1.
How have I tried to solve the problems on dark? I’ve used the desert map since I seriously wanted to try Yune out, and I only have one healer whose job is more to restore than it is to actually deal damage. Harsh commands are very useful on dark so that Temari and Sabo. Atk can be used against you. My specific map has SMEirika, Rath, Louise, Cecilia, and Veronica (who will be switched out for Priscilla or Clarine when merged) and Yune. Throw out as many debuffs as you can, and build your units to be as offensive and advantageous against the mythics especially. If you’re running blade tomes, it’s advised to have multiple sources of Harsh Command (HC+ especially in cases of panic) or restore. In SMEirika’s case, her nullification of debuffs means you can use Hone Cavalry on her since she doesn’t care about being panicked as much. One other weapon to mention and unit I consider to be nice is Lorenz’s Conch Bouquet on V!Titania, and Summer Sylvain. Their weapons have great stat boosts compared to the other inheritable red tome options for a cavline, and it’s dual-phased which is pretty decent for staying a threat on EP against Eir and Peony. If I could get a Sylvain merged up slightly, he’d have been a great substitute for the Cecilia I’m using from his weapon and statline alone.