I think the midlength is the way to go, just keep it eggy and foiled not "short longboard/ magic carpet style". Kadvangs looks lovely and is a great reference
I'd definitely get it with a box and plugs, you can always ride it as a single that way too. Personally I wouldn't though!
In broad terms, a single fin will tend to pivot in turns a bit more whereas a 2 +1 will allow you to draw your turns out more and carve and project out of bottom turns which helps cover the ground you need to in sectiony or mushy waves.
Also a single fin tends to lose speed through a cutback whereas a 2+1 will drive more through the turn and maintain speed which is better in those kind of waves. Trust me a proper wrapped cutback on a 2+1 egg feels amazing!
Finally, if you need to pump to make it over a flat spot or past a walling section, a 2+1 does that better.
Using your rails more and having a flexy single fin goes some way to improving these areas on a single fin but the 2+1 will perform better.
This is my Howard special
6'10 and so much fun in shoulder plus. It's only downside is that it doesn't generate speed that well since its roll into panel vee which is why i'd suggest some single into vee'd double in the bottom of yours.
This is the 7'10 I posted above which has concaves and is VERY fast in mushy beachbreak. Surprisingly loose for it's length too.