Is it somehow possible to sync the achievements?
I have already played around 30 hours on steam and want to continue on GOG, but since I'm an achievement hunter, I'd like to keep my achievements. Is there a possible way of doing this?

Steam is an entirely separate company than GOG.

Why would something on Steam carry over to here?

Why would games you own on steam be able to be redeemed in your gog library?

Honestly, Why can't you work on the achievements, here?

If you want to cheat to get all the achievements, I would assume you can find those on either Google or NexusMods.

Because the companies who own the games agree to do a promotionl deal with GOG.

I would like to continue playing here, but then I would miss achievments I already earned with this save file on steam (because they dont activate retroactively).

If you import a cleaned-up for NG save, you will recive a few achievements when you start a new game: enemies killed, locations visited, gold/equip obtained.