I'm using Libro[0], which is essentially a DRM free Audible (including the membership!). It's a bit of a hassle to sign up without a North American Credit Card (you can only buy their membership with one, but you can gift it with a CC registered anywhere else, so I gifted myself one).

It lacks Audible exclusive titles (which are shockingly many!), though.

Before that, I used to strip the DRM from my audible books (easily doen using ffmpeg)

[0]: https://libro.fm