Hi James,
Thanks for posting this. I can confirm your workaround seems to work. Once I manually copied and renamed the default 'arcgispro-py3' environment to my user profile, I could indeed install new packages and make the change of the environment stick in Pro, instead of being thrown back to the default environment.
So I can now finally get to work.
I did not remove the 'read-only' property of the folder by the way, just copied and attempted to install a new package in the copied environment. You are correct about the 'proenv.txt' being generated as well, I saw that too. I haven't tried to install the Spyder package by the way, I just need pyodbc for now.