There are 40 separate downloads for the game. Is this the best way GOG can do for the offline backup game intallers?

Short answer is yes. I've seen people suggesting the files to be optimized but that is not ralated to their number/size. If you are interested to read in depth answers you can search in the main forum there was a thread not long about it. But in short - one big file and a sudden internet problem or file corrpution would mean you are downlaoding 160GB again. The size limit I think was related to different file systems - some can't handle larger files (>4GB)

I am now on 23 of 40 lol

Not many will agree but imagine for example if GOG had only 2 files of 50GB each and suddenly something happened: such as your connection or something with the gog servers...Then you had to download 50gb again.

You could try to download the game files via GOG Galaxy. Go to the game in the client, then choose Extras - there should be an option to download the install files. Other possible ways include command line software like LGOGDownloader, which is however a Linux tool, that is what I use to backup my whole GOG library to my NAS regularly. If you are on Widows, you should be able to run it if you install Windows Subsystem for Linux.

Well... I think GOG could actually provide a choice for a player - download 40 files OR 2 files 50 GB each. Then It would be a player's responsibility in case of connection problems.

Everyone knows what will happen if your 80GB(because 50 wont do it for 2 files)gets corrupted - GOG will be to blame first no mater what the reason is. So this is not a good idea at all. I think the most reasonable thing is to use a program to download them where you can queue as many as you want for example.