Audiobookshelf works great and is very easy to setup using Docker. I get most of my audiobooks through Downpour. They have a subscription that gives credits to redeem for audiobooks similar to Audible, but they are drm-free. I download the audiobooks and move them to my NAS that has Audiobookshelf running on it. Audiobookshelf has a web client and apps for Android and iOS (TestFlight beta).
I’ve been using Runbox for a couple of years and agree the web interface is lacking. There were also multiple significant server outages last year, but it has been much more reliable this year. As long as you don’t use the webmail often, Runbox is a good affordable email provider and is great for families due to the low cost of sub accounts (additional users).