bicycle.journey you’re an ideal candidate for a type of bike called a minivelo.. They’re compact, rigid frame bikes built around 20” wheel sizes. They’re most popular in Asian markets, particularly Japan, where storage space for bikes is tight, as in urban apartment living. Having shorter wheelbases than full-sized bikes makes them easier to stash out of the way, but also easier to manage up stairwells, too.

Minivelos don’t have their own forum here, but have a subculture on the Folding bike forum, so posting an inquiry thread there will get you on the way of discovery.

To get you going, check out the Polygon Zeta at BikesOnline.com:

https://www.bikesonline.com/2021-pol...inch-city-bike

We often find minivelos in the budget bike segment in the West, so often, as with the 28lbs Polygon Zeta, they’re not built to be light, bit I know higher quality minivelos are out there. I suppose it could be fun and educational to upgrade and customize one with weight loss in mind, so that’s another pathway if you’re interested. Just looking at the Zeta, I’d guess 3lbs could come off for not much money or hassle, but still, 25lbs is not really lightweight; hoisting 17lbs up two flights of stairs would be nicer.

Here’s a carbon fiber framed Java at 9.8kg (~20lbs):

https://www.mrbicycles.com/product/java-mini-velo-bike/

Anyway, investing some energy in exploring minivelos is probably worthwhile for you. Best luck!