I getting error, that my saves exceeded all memory assigned GoG for this game? (1GB) it is for real or some bug?

use cleaner from the nexus. it will both solve loading times(it will still take longer in the later chapters, but will become almost tolerable) and fix save file sizes.

upd. and yeah save file bloat is real as it stores lots of unneeded data. mostly corpses and their location along with their loot, etc. how many corpses you have left during the game and how many items you dropped on the ground? that's the root cause of the issue.

but I have only one character (4 saves, 5quick and 5 auto) and already more than 1GB?

every single save gets bloated and yes, it is that bad. use cleaner or suffer.

as far as I can tell no mod for unity mod manager disables achievements cause they do not use the game itself to inject.
I would leave you links, but GOG forum rules do not allow that afaik.