I use Downpour for Audiobooks. It is similar to Audible where audiobooks can be purchased individually, or there is a subscription that provides credits to purchase audiobooks. The audiobooks are drm-free and can be downloaded. I have not found a way to automate the download and transfer to my Audiobookshelf server, but I don’t mind doing it manually considering I average around two or three audiobooks a month.
I use Radicale for my calendars, reminders, and contacts precisely because of how minimal it is. It has been very reliable for me and is very easy to back up and restore since it is just files.