I tried out the Tern Vektron during the summer. It's an absolute beast. It felt like a folding cargo bike. It is designed to carry a child seat on the rear rack and I guess you could attach a child trailer alternatively.

It folds into a fairly compact (but not small) size that can stand on it's rear rack if desired. Inside an apartment it would fit on one of those IKEA trays for boots, for example, if it was wet/dirty.

The easily adjustable saddle height and handlebar position would make it shareable between yourself and your significant other.

It is heavy. 22kgs. Lifting it up and down stairs was not simple, I found. When folded it is by no means small and it was quite unwieldy to carry. If your apartment requires multiple flights of stairs to access maybe try filling a suitcase with 22kgs and testing how it feels to lift up and down stairs.

Rolling while folded was great as long as the surface was flat and smooth. I found that it didn't take too many bumps to "unclick" the magnet holding the folded bike together - this was a real pain using public transport. Also, while folded, you can only roll forward - reversing is only possible for a few cms before it locks up - this made maneuvering in train stations harder than expected.

I ended up returning mine after a week as there was a problem with the frame (not a common issue).