Reading this I did a brief search and Kobo looks sort of interesting in offering combined book/audiobooks. A subscription for 1 book a month at $9.99 with no option to buy more credits means I'll be spending a bit more but the extra convenience might be worth it.

https://www.kobo.com/us/en/audiobooks

EDIT: Audible's media integration and volume levels have been sort of broken on Android for a year now so I've been vaguely meaning to switch and this is just a prompt.

I don't personally listen to audio books, so hopefully someone can chime in with some helpful alternatives?

From a quick search though, it looks like https://libro.fm/ is where I would start looking with an Audible alternative as it seems to align with my ideals and has a decent selection. There does seem to be a good few options though so have a look yourself if nobody has further suggestions :)

I don’t have an answer for a general audible competitor because I too use audible. However I do purchase directly from one studio that I’m a fan of, Soundbooth Theater. There may be other groups with a similar set up, but I’m not aware of any (nor have I looked though)