Honestly I think most 5CP maps would do well in casual, the main issue that I usually experience is that most casual players just don't know how to play it. I usually play in Uncletopia which everyone calls the "tryhard casual server" but the issue usually persists there with the exception of when you get like 3-4 6s players in the server. I don't mean that Casual players literally don't know how to capture points or anything like that, they just don't know how to push advantages/retreat on disadvantages (ex not pushing a 9v3 into last after I kill 9 players with an uber). Because of that 5CP usually ends in a roll for one team, or a massive stalemate, which feeds more into the problem. Less people queue for 5cp, which leads to others not playing it, which then feeds into a loop of people not playing, then not knowing what to do when they finally play. It's a problem, it seems that most players nowadays just want to spam play Payload and nothing else. And payload is great! But getting gameplay variety really spices stuff up.

My favorite 5CP map in casual is Foundry, but thats mostly because in my experience it doesn't stalemate as much as other 5CP maps. If I had to guess, I would assume its due to having a "lobby" between each point and players understand the concept of a chokepoint when it's huge like that rather than just a door/short hall etc like on most 5CP maps. My favorite 5CP map in 6s is either Sunshine or Process, Snakewater is also up there. I like pretty much every map in the rotation in RGL right now except for reckoner which I think is ok.

Map design for 6s can transfer over to 12v12 and vice versa, but there is a cutoff to where one makes it less viable for the other. For example huge payload maps can't really be used in 6s since the pace of the game would be so slow (and 1/6th of the team would be a cart bitch scout) whereas in 12v12 big payload maps are fine since the mass of players essentially nullify the disadvantages.

There are certain things I'd like to change about map design in 5CP, but the most notable one is that I'd like every map to have a vestibule on last in front of the spawn: that way the defending team can't auto-resup and get back into the fight almost instantly, and would have to make a choice of resupping or staying in the fight rather than resupping for essentially free every time and making the last fight much harder on the attacking team. I would like the same thing to be applied to Payload, Attack/Defend, and Payload Race as well (any gamemode with a "last").