Interesting. I have 3tb od Games installed on my Gpd Win Max 2 and it takes super long to start GOG Galaxy.
Especially when the Internet connection is bad.
Also the feature I am missing most is to back up my complete offline installer collection and to keep them updated.
I don't have so many games installed, but surely Galaxy does weird things at the start... it seems always to reload the entire library from scratch... some kind of caching system and a background synch could speed up a lot the things...
Anyway, Steam and Epic do the same... they always have to reload all the images from scratch at the start, plus all the store pages stuff... Epic is the worst, it's almost impossible for me to browse the library from there, it needs a lot of resources and internet band every time, just to reload the thumbnails...
On Galaxy this is ten times faster for me, and I can scroll in a single view all the games I have in all the 3 stores.
On the forum there are a lot of complaints about the integrations, like missing games, crashes or infinite loadings, but for now I had no issues.
To backup your offline installers just use LGOGDownloader, it's only for Linux but it's what you need to do the job. It can get even the extras, and you can launch it also just to keep updated the stuff you already got. Just have a look at the command-line parameters and you'll see how powerful it is.