Yeah, Google didn't put much thought into file transfers between a watch and phone, so the options are really limited. I guess they assume any data transfer between a phone and watch would be relatively small and not large files.
16MB is about near the limit. I've found the transfer is pretty reliable at 20MB and below but above that and it may timeout or not transfer at all or just take forever.
I know there are other WearOS apps that focus primarily on custom audio playback. They might have a better solution than mine. Wear Casts is primarily for podcasts and the file upload is secondary.
For example, this app is all about audiobooks:
https://play.google.com/store/apps/details?id=com.turndapage.navbooks
Wear Casts does let you upload custom audio files and it will keep track of the position as well give you the ability to change the playback speed.
The tutorial titled "Local Playlist" does into more details on how to upload custom audio files to the watch.
The phone app does allow you to upload audio files to the watch but it works best for smaller files. For larger files I recommend Wear File Manager.
Wear Casts is probably not the app you are looking for. You want to download Player FM or PocketCasts which are podcast apps that run on the phone. I know Player FM allows you to control playback on your watch.
https://play.google.com/store/apps/details?id=fm.player
Wear Casts runs on a watch only.
You can cancel your purchase here: