I already own the series on steam. I have no issue running it there, but my main reason for going to GOG first for titles now is the policies vs each other. If GOG goes out of business, I still have my games, if steam decides to lock my account for any arbitrary reason they decice, or goes out of bussiness, I lose access to pretty much anything that wasn't a DRM free indie.

The best is even to buy at Slitherine, as their exe is even more drm-free than GOG'.
I really don't know why you bought at Steam', it was so much repeated since long that Steam is a drm.

I didn't own any other slitheriene games at the time and it wasn't on GOG nor did I use it then. I'm not going to buy two $80 games twice.

Other than at publisher request, GOG is DRM free, you can literally copy the folder somewhere else and it will work. Like some indie titles on steam too such as KSP or Rimworld. Rimworld knows it's the steam version, but doesn't require the steam API to function. Same with KSP.

Yes. But if GOG dies, you will no longer be able to patch your game, even if the devs company is still alive.
We have bough the games at GOG', not at the devs. Devs are not contractually compelled to do the special patch of GOG games if GOG desappears, and they may even not be able to do this without GOG devs.

If slitheriene allowed me to use my serial number to register on there and be able to download it direct, I would, but they don'r. Still not going to buy them twice at anywhere near full price or even half off.

They do. In fact they even give complementary Steam keys in addition. There's a page on Slitherine you need to look up and you can add purchased games. Afterward, you can download both game installers & patches.