Dogmaus: As for the biggest drm-free store, I don't have the numbers to prove it but I feel it might paradoxically be Steam.
If you are just saying DRM-Free and not using any quality or non Indie arguments, then surely it would be Itch.io that has the greatest collection, and certainly they also qualify from a direct download aspect. Steam can be problematic when it comes to DRM-Free, with as you indicated, it not always being clear what is or isn't some kind of DRM-Free.
Personally, while I recognize Steam has a large DRM-Freeish collection, I don't see it as a proper DRM-Free direct download store. And while some perhaps many games are relatively easy to make DRM-free with a simple change, some require a bit more effort and all require backing up a folder, which isn't much of an issue with a small size game, but large games with thousands of files can take very much longer and perhaps cause AV issues. Then there is needed dependencies and registry entries in some cases, which aren't always obvious. Worst of all though, is the need for the game to be installed in Steam to get any update, DLC, etc ... which means backing up all over again.
So really GOG is the largest mainstream games direct download DRM-Free store, and so easiest and quickest to use, without a doubt ... certainly for archiving & updating your archive.
P.S. While I have many games at Steam, I am yet to backup any of the DRM-Free ones. As I have better things to do with my time, and aren't that desperate. Especially as it would involve investigating what is or isn't etc, and then zipping the folder to save on archive space. Life is too short.