@miyako19 is all of Japan really expensive? I know Tokyo is. It seems like they are doing just fine living in their home town (I think it's on Shodo Island). I mean they have a normal sized apartment. They can take Chi to school (this is probably subsidized but still). Takagi-san doesn't work. So his salary seems to be fine. In most countries a PE teacher has to take special education and has a bachelors diploma from a college/university. So their pay isn't lower compared to other primary/middle school teachers.
Also what plot? This manga is just random slices of their life together. I was thinking the baby would just be another cute character and somebody else for Chi to interact with and be the older sister for. Sure you can do a time lapse, but babies can be cute too, example: https://mangadex.org/title/48216/married-couple
I suppose what I am saying is that it wouldn't hurt to have new characters. That aloof looking boy who hangs out with Chi was a good start.