Pie plate: I was actually gonna suggest that, based on your earlier pictures. I'd taken off my other bikes, for style, but then added to my Dahon after the chain, only once, jumped off the big cog and into the spokes, tearing up a couple just enough that they eventually broke. Found a disc and put it on (bike shop stocked them, now they don't). "A properly adjusted derailleur will prevent that." Not always. May have been a bump. May have been because it was the original "compact" Dahon derailleur, had always been problematic, especially shifting onto the largest cog, which was the biggest jump. New derailleur (conventional style) fixed that. I prefer the dork discs that are clear plastic and minimal size, but you go buddy, rockin' that 70s vibe!
That Eco monostay rear frame is nice for a belt drive, you don't need a frame that comes apart in the rear triangle somehow, however you need some sort of belt tensioner, unless it has a frame with horizontal dropouts that allow tensioning a belt or chain, like the bikes with internal gear hubs have.