I have rats in large opened out SRS sized cages, and the HLD rats still manage to get around fine (their upper bodies gain muscle and strength to compensate), and I think it helps them live longer than they might if I consigned them to ground floor living. There's still a good amount of floor space (95 x 65cm) if they choose not to though.
How about a single level critter nation
(or even get a second hand SRS but build it as a single level)? Or with a bit less floor space but cheaper, a Zeno3