Thanks everyone for the thoughtful feedback. I haven't bought anything yet, so I'll definitely go with a Mantis and not worry about size/expansion/having to pick smaller modules/etc.
I was mostly concerned about utility/basic type modules and if I was missing anything obvious that would work well and compliment what I had in the case already, or just general essentials. Or if what I had didn't make any sense at all.
As far as the Benjolin goes, that seemed like a good option for an out there crazy oscillator for noisy stuff, but I haven't looked at it enough. Maybe I'd be better off going with a simpler 2nd voice/oscillator to start. I'll have to look at it more. Same goes for the Bastl filter. I didn't look at it too closely or get too deep into seeing what other filter options are available. As for the Intellijel Mixup, I wonder if there are better options for a case this size or if there is a mixer with 1/4" outputs? I'll have to look into that more. I know everyone loves MATHS and maybe I should consider that. Befaco Rampage seems like another useful module that I've messed with in VCV Rack. And I'm still really interested in morphagene/ahbar, so I'll have to read other threads as to what people say about how those two compare.
As Jim said, I'll probably buy as few modules as I think I can to start out. Good advice.
Thanks again for all the comments, I appreciate it!