I haven’t tried it myself yet, but you can get yeast improvers , a powdered ‘mother yeast’ that claims similar results to sourdough.

I have a starter in the fridge that I only use once every two or three weeks, and have not had any mould problems; perhaps you just have to be only a little less lazy to keep a viable one, and feed on that sort of a schedule?

I agree though, that making sourdough bread can be a nuisance time-management-wise until you find some sort of rhythm that suits you.

I don’t think I’ve come across that before, but I’d say it depends on what is meant:

• I don’t know what that thing is.
• There is a thing, but I don’t know what it is.
• There is a thing such that I don’t know what it is. I.e., I do not know what all things are.

There may well be some other ones, but I don’t know what they might be.

Oddities and Curiosities of Words and Literature by C C Bombaugh, one of my favourite reads, feels like it might be an obscure book.