You can disable all of Galaxy's automation and maintain full control over your games. It can be configured to not auto-launch on system startup and only run when you explicitly start it. You can turn off all the automatic game patching and downloading, tracking time played, achievement, friends list, chat, in-game overlay, notifications, GOG Store, etc.
On my system, with the very minimal "everything disabled" configuration I use for it, it takes about 1GB of disk space and uses about 322MB of RAM and 0.1% CPU utilization while idle. (But I still keep it shut down when I don't need it.)
If you're simply looking for a better way to download those large multi-part installer files, a download manager browser extension like DownThemAll (
https://www.downthemall.org/) may be a better option. But if you want any of Galaxy's other features, you can configure it to run in a very minimal way that still allows full manual control of updates and only launching when you need it.