I also purchased my original account on Steam (in my case, in 2016).
I was able to download the ED Launcher from my Frontier account page, and install using that.
It works fine without Steam (although it does notice if Steam is running and tries to connect to it -- I just make sure Steam isn't running).
Thus, I was also able to order the Odyssey expansion from the Frontier store, since I'm not stuck with Steam.
You might want to try it and verify that you can run without Steam. If so, you're good to go I'd say.