There's this great thing here in Russia, called "the Ilya Frank method"... It's sort of... well, the German (but it's for many different languages) text is separated into paragraphs, first you get the text with inline commentary (translations and explanations) in parentheses, then the same paragraph is repeated without the commentary. I got several books for my mother, well, mostly two collections of jokes...

I don't know, maybe the same thing is in your books, or maybe it exists in the English-speaking world. I just think it's a better idea than simple parallel texts.

Anyway, I'm actually pretty much certain that I'm not going to find what I need in Welsh. So far there's been two paths to language for me - via very specific genres of books (optimistic fantasy and historical fiction - make of that what you will), and very specific genres of Youtube videos (gaming - although I'm much less enthusiastic there than I used to be - and some others, like history popularization). Welsh language is in no state to provide any of those, really...

I started off with this knowledge, but it has turned out more demotivating that I expected...