First thing that comes to mind are the TF2 items that weren't supposed to be in the GOG release to begin with. Are they removed if I upgrade to Re-Elected? Any other downgrades like missing music and such?

When loading a old savegame it says that DLCs are missing that were previously in the game.

However, the Re-Elected version is supposed to have all the content of the original version including all DLCs, my suspicion is that the DLCs just don't exist anymore and are integrated into the main game.

I didn't play far enough to find (or miss) something that was part of a DLC.

From what I have read, we lost access to the supersaints and thank you pack, borderless windowed mode, and possibly yes, the TF2 pack. Re-Elected is a direct downgrade to the game, not an upgrade.

TF2 pack is still there, don't know about the other two.

The problem with Hitman was, that single player content was drm locked. This is not the case for SR4.
You can still play it without EOS, so still drm free. The EOS is only needed for multiplayer, so those who want can finally play with those who bought the game on Steam or Epic. If they used XBoxLive or Steam even, the outcry would have been louder.
Also you can revert the game back to the original version released in 2017 with all it's original content. And yes - that version crashed when you clicked on buttons with online functionalitites, that is nothing new.

All summed up I would say, they did learn.
Would have been nicer tho to just have two separated entries in the game library.

How can I do that, please?

On the game page in Galaxy click that options button in the upper right, go to "installation" and "configure".

Then
1. disable automatic updates and (this you can do during the installation already)
2. select the desired version. Ther are more available, but I would stick to the latest from 2017.

My screenshot is from the german Galaxy version, it should look just the same in english.



What is a bit annoying, is that the preset for automatic updatesis "on". So when installing from offline installers, Galaxy will instantly try to update. If one doesn't stop it fast enough, he has to revert as shown above.