Have you considered using traditional Japanese futons rather than a conventional mattress? You can just fold those bad boys up and bingo! floor space returned!
The most fun thing about tiny homes are all the little hidden nooks and crannies you can store stuff in. Storage furniture is king!
Plus I recommend having at least one large window because it saves on burning electricity in the winter from needed light. That and placing big ass mirrors on the wall make the space feel larger and brighter!
And literally any reclaimed items you want can come from Buffalo Reuse etc.
Futon the best
There are so many ways :D idk why everyone isn't living in tiny houses!
As for the roll up mattress, idk my back isn't so great :/