An acquaintance who lives in Germany and has an infant said that pretty much everyone there uses a 3-sided crib attached to the bed. I searched amazon.de for "Beistellbett" and... they have so many options! And it looks like they're meant to attach to frames with slats instead of box springs. Sadly, I don't think there's a way to have these shipped to the US, and they may not meet US safety standards.
I'm still not sure what we'll do (and really know for awhile, I guess!)... Leaning toward putting a queen mattress on the floor, with the possibility of getting a twin mattress (separated so there's no little gap for baby to fall into) for my partner to sleep on if it's too crowded (and for baby to sleep on later).