Hi there, welcome!
I’m not really sure I fully understood the request: you expected to use your Ubuntu GRUB with a Garuda line there?
If this is the case, you should do the other way round: use Garuda’s GRUB and Ubuntu will show up there as well.
This post could be similar to this request.
About the boot sequence order, that’s a different problem. My only suggestion is to avoid Grub-customizer, which is quite common in Debian-based distros, because it does not work well at all here.