Spinney is a nice word for a smallish gathering of trees, alongside copse, coppice, etc. I’m not aware of a term for one specifically in an open field, though.

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.