I use podcast addict in much the same way. I can control it using my WearOS watch or even just via the Bluetooth controls of my headphones. I use podcast addict mostly for podcasts (obviously), but I also have (a very manual and kludgey) RSS server at home to feed me and the rest of the home audiobooks via podcast addict. I’ve started to move that listening more to Plex via PlexAmp on my phone, which is also controllable from my watch. The Plex audiobook experience isn’t perfect, but it is a lot easier to manage.
I haven’t found a need or advantage to an external player yet. But, I have found some codecs that the Plex app has struggled with, which might benefit from an external app. I haven’t had the issue in a while though and didn’t think of using the external player then, so no guarantees.
No, I don’t use the podcast feature on the Plex. But I do use Plex for listening to audiobooks. Just be aware that MP4/m4b cannot be in the same library as mp3s.
The other podcast thing is a solution totally outside of Plex. It is running on the same machine though and accessing the same files. dir2cast + webserver like Nginx or apache reads a directory of mp3 files and builds an RSS feed out of them. In some ways it works better than Plex because it’s simpler on the user side to listen offline as long as you sync the feed at home. I tend to do a separate feed for each series or author. It’s a bit fiddly to get setup and adding a new feed requires a bash one liner and editing some HTML after the files are sorted and named perfectly because podcast apps have some funny limitations when it comes to actually grouping, sorting, and displaying metadata.