[...]
Since update 15, Epic wants to re-download 20.6 GB, if I'm not mistaken that's the exact size of the Legacy folder I deleted to free up space.
I managed to circumvent Epic by launching Elite with a python script that opens a browser window with a basic HTML page that has my auth code, which I copy and paste into a python text box, which then sends that info to the Frontier Launcher, which only updates whichever version is selected.
Since the Launcher Update, I'm not sure if the python script is actually necessary anymore, but the two times I've tried to open the Frontier Launcher without it, it has asked me for a product key, so maybe it is.[...]