I would like to start a new game without Relics of the Past and experience the original balance, as I got them together. I can't find a way from inside Galaxy. In-game menus and game folder don't seem to offer a clear way to do this. Any help?

I can not answer that question with certainty, but from what I have seen there is no difference which would justify playing it without the DLC if you already have bought it.

The point of the game is kept even when jumping between versions (see below) and there are no differences which I have spotted (but the DLC do list a lot of enhancements - but nothing I would like to avoid by playing without the DLC).

Even on the contrary - concerning Linux it is now a 64 bit binary working out of the box on Ubuntu 18.04 LTS, Eoan 19.10 and Focal 20.04 LTS, while the old in development Demo on itch.io and the GOG version without DLC are 32 bit only and need a little work (as described in my post "Demo version, missing 32 bit libs and DLC with 64 bit binary").

So I would only recommend to play the game incl. DLC (for those who have bought the DLC).

But if you really want to get the impression yourself: just download it from the webpage
    https://www.gog.com/account
as there are two separate install scripts for the base game and the DLC - so installing the base game in two different directories and only installing the DLC to one of them is easy - so you can switch (I do it as a habit - but till that day I never saw a reason of playing a version without DLCs - but I avoid those which are known to bring in bugs annoying gamers ...).

I have the problem that I will not use Galaxy as I don't want these functions (one of the reasons I am not using Steam - and will not in future) and don't get the development version via the Download page ... which would be nice if that will change in near future.
But thus I don't know about a possible workaround with Galaxy to install the base game only.
If one wants it one may purchase the base game first and install it - then copying the install directory - and later purchase the DLC and install it ... that should work also with Galaxy (renaming directories etc.).

Hope this helps.

Vielen Dank für deine umfassende Antwort. I was travelling and onlz now did I see it. Installing without Galaxz is actuallz a great idea, it´s weird how used to it I am now that I didn´t think of using an offline installer. It probably won't make a big difference but I'm loving the game so much I want to see how it developed and will also try the Itch.io demo.
Hopefully we can get Galaxy for Linux too, it would only be fair to all old users always loyal to GOG and Linux. I sadly only have a Windows machine atm but not without many regrets and I'm terribly missing expecially Ubuntu but also messing around with other distros.

Apart from what JMB said, if you want to uninstall a game DLC from galaxy, you have to right click on the game and choose settings, in the configure game section, you can mark/unmark any dlc you have from a game. It works like this on the pre 2.0 galaxy version, if you have 2.0, it might be different.

Super! It worked and my game is reset to vanilla. All progress is lost in the process. Thank you!

Maybe you can recover the cloud save. I didn't think of the possibility of saves losing compatibility when downgrading the game, but the save system may break for people trying to remove features instead of adding them, sorry. This method hasn't given me any trouble with other games, hope you can retrieve what you lost somehow

Sure, I was just warining others, I don't care about the save personally, I am happy that the game reset itself.