I’ve been building a podcast app that has some options to increase the font size (see the images), though it may not be the complete solution you’re looking for.

Local on-device player that allows you to enhance and fix the audio. Plays local files, too.

I tested with Accessibility —> larger font size and it seemed to work but may not be everywhere.

If you want to test it and find places that are problematic, happy to try to fix them.

https://Nuotit.org (I haven’t started to market it yet)