I have had some success on Windows just by changing the .txt file extension to .md. Windows throws a popup complaining about potentially breaking the file but I have not experienced any breaks yet.
This wouldn’t address the issue of ongoing cross-platform compatibility between .md or .txt files but might help with transitioning from .txt to .md.
I have not tested any text files yet beyond fairly simple text so I don’t know how any coding or scripting might behave when changing the extension. I’ve made copies of all my files before making any changes, just in case.
Like many of you I have been moving away from proprietary environments and file formats for years now and working towards all text files.
I really like the tool Bulk Rename Utility (free) really useful for these types of tasks. It’s very powerful and provides a preview of changes before they’re performed - use with caution. Or a script to parse files changing the extension would make light work of it.
Here’s a screenshot from the Bulk Rename Utility interface settings I’ve used:

Jacqui
p.s. please be gentle, it’s my first post