Sioned has just released a book for adults “Madws”, which is a fantasy novel. It’s a cracking read (if you like that sort of thing), and she has a lovely “voice”.
Also, I recently read “O Ran” by Mererid Hopwood, which I had picked up in a second-hand bookshop. It won the Medal Rhyddiaith in the Eisteddfod a few years back, so it’s not too long, and she writes simply but beautifully (as you’d expect!)
Thanks for the suggestions!
I have learnt, from experience, that novels for adults that have won prices are too hard, but novels for young adults that have won prizes are fine. 
Basically, I’m not yet at the stage where I can comfortably read a book that would make for a great reading experience for an adult L1 Welsh-speaker. Less varied language and less of the less-used vocabulary works better for me at the moment.