I posted a thread about building a fat tire full-size "mini", and am following this thread with great interest!
I like the simplicity of the spine frame, and think that's a great option for quicker and more efficient building.
It would seem to avoid the need for mirroring your frame spars, which could get time consuming in terms of fabrication and fitment. If you don't mind asking, where did you source your tubing? Is it new stock, or sourced from scrap (scaffolding, etc.)?
If I could find a used ATC front-end and frame neck, that could simplify my build and get me in the neighborhood of where you're going. I really like the single-sided look, though share concerns about getting the axle carriers strong enough to resist axial loading (I weigh over 200lbs, so that doesn't help, lol...). Not that I don't think you'll have it sorted, just trying to think of ways to make my first mini build easier... Would like the fact a front brake, hub, wheel, etc. would already be on an ATC unit as well...
Anyway, looking forward to your next steps!