I'm considering stepping up from my Phantom 3 to a hex or octocopter for more flexibility on cameras, gimbals, etc.

This would be used with cameras larger than an action-camera.
Something like a ca. 250g camcorder.
Ideally I would like to be able to ultimately mount a small mirrorless camera (ca. 500g max.), but that's definitely a "stretch goal".

I am open to either a pre-built or custom DIY design.I have pretty good fabrication and assembly skills, so I should be able to handle almost any task in the DIY category.

One thing I want to avoid is having FC software with any kind of geo-fencing. I think that pretty much pushes me to the DIY (?).

I'm leaning toward a fairly large frame/propellers; at least 500mm/10inch or even up in the 900mm/18" category mostly for reduced noise.
I see there are a lot of frame kits out there (esp. in the 550'ish range), but I have no idea what specifics to look for (or avoid).

I would be grateful for any suggestions on the frame, motors, esc's, FC, FC software, etc.