I'm pretty happy with the aquascape situation right now. What has happened is that my engineer goby had almost entirely hollowed out the back of the tank of 3 - 4 inches of substrate, making a bunch of huge caves and piling most of the sand in the front and the right side, and he seems hospitable enough to let all of the fish sleep in there. I really liked this setup because it opened up most of the front for viewing, but there's enough secure hiding spots that all my current fish becomes nice and complacent within one or two days of introduction, even the skittish ones like the goby and foxface. As soon as they get into the rocks they're completely invisible. The tank only has one and a half sides you can view from excluding directly above so that adds an extra layer of security too.
Regarding the butterflies, it's mostly the rarity factor that makes them desirable. I've been pretty wary of butterflies but as of late I've heard constant good things about the longnose in particular in terms of both hardiness and reefsafety, in combination to my lfs getting the first one in a very long time makes me want to give them a go(given it's a confirmed chungus and feeder ofc). I think it'll be a pretty easy rehome to do too, never ever seen one in my area.
For the horseshoe crab, I've been developing a full on game plan for tackling them... it's not complete and it involves taking some fish and corals out, but eventually it's something I want to keep after reading enough literature on them. Right now the urchin collection takes a little precedent though.
The lamarck is just an impulse remark lol, it's pretty cheap at my lfs rn but I've seen what an adult looks like in their display tank. I won't get it unless my mum makes me.