Continuing the discussion from Yet Another Apple Music Folly:
I stopped using Apple’s Podcasts app many years ago for a few different reasons.
These days, I use Mimir on my Mac and PocketCasts on my iOS devices.
I don’t know about “enhanced m4a audio files”, but at least one of the podcasts I subscribe to has chapter markers and I see them in PocketCasts.
I use PocketCasts both on Mac and iOS. Happy with it I have to say.
No idea what “enhanced” means, but I use Overcast because I prefer the way it handles fine-tuning which episodes I download, and I can use its website to upload the occassional non-podcast audio file for listening later.
Just a quick comment that the ability to upload your own content with Overcast is restricted to subscribers ($15/year). I’ve uploaded audiobook content to Overcast myself, but none that include chapter markers.
Castro is another podcast app that allows side loading of your own content, again for subscribers ($25/year). Again, I have side loaded my own audio content, but nothing with included chapters.
A last podcast app that can sideload content that I have used is Downcast, which has no subscription but costs $3 one-time.
All of these I’ve used successfully to sideload .m4a audio files.
Paying customers can sideload in PocketCasts too.