Given the amount of discussions going into romance stuff: less romance content and more friendship content. And generally have things with companion move more slowly. Gale is best friend with me on day 2, Shadowheart spills her beans about her goddess merely a day later, Astarion tries to bite me on day 2 too and wants to get cozy on day 3 , Lae'zel isn't far behind. And I play a goody two shoes most of the time, so Astarion and Lae'zel should be far more unhappy with me for a longer time. Shadowheart should be more paranoid (I miss my edgelady from EA tbh) and Gale should be a bit less trusting too. He seems to be a very open person, who likes a good chat and I'm totally ok with the Weave scene, since that is Gales thing, but I get the feeling Gale is BG3 Varric: you just have full approval by not being a total asshole all the times.
The only ones making sense are Wyll and Karlach. Wyll has no option, since Mizora just appears in your camp like the annoying Karen she is and Karlach is just that bubbly person, who is very open from the start - though I would say, she doesn't have much choice but be open about herself too, since Wyll will confront her.
Halycon: Halsin is just the perfect victim for Orin: if he gets abducted, everyone lives, which is not the case with some of the others. If you trigger the abduction scene early, you don't really have him in the party for long.
But you are right, you can send the others away, you have the option to kill other companions during their story. You can of course botch the portal quest for Halsin and he will be stuck, but then the curse isn't lifted. This time in my honour mode, he didn't even announce to come with me in that creepy way, where he tries to flirt with you, the game just did put him in my camp and that was that. At least Minthara did kick him out of his tent, whic hwas kind of hilarious.