And you can find an aggregated list of grub files for various distro's at aguslr's multibootusb:
Re:Rufus: Reliable USB Formatting Utility
by thomastjeffery on Friday May 07, 2021 @11:20AM
(#27076873)
You can also add toram to copy the entire squashfs root to memory. This way you get better performance, and don't need to keep your USB disk plugged in.
You'll also want to make sure you've insmod'ed any modules you need for your hardware as necessary (storage, USB, etc.)... unfortunately documentation on these is poor, and some modules are mutually exclusive, so have fun.
Best of luck getting a robust boot menu script going. Expect to spend weeks if not months on it if you're new to GRUB scripts/components.