Help Building a Menued Mult-Boot USB Drive (Grub2?)

Greetings and thank you in advance for any assistance you might offer.

I am trying to build a bootable USB drive that can work either from Legacy CSM or EFI PCs that will have multiple operating systems displayed as selections in it's menu and I'm guessing Grub2 is probably best for that but I'm a total novice at best for Grub2. I think it would be very useful for installing and maintaining systems on many PCs.

I've partially succeeded by installing Grub2 to a test USB drive which will boot in both modes but I'm failing to get a menu for selecting the 2 operating systems on this pendrive. This one has a Hirens Windows PE type system and a Slackware Install system, and I'd like to add Liveslack as well once it works.

Currently if I select UEFI boot from my Boot Order Menu the PE system boots but directly with no options. So far I can't get the Slackware Installer to show up let alone boot. My totally novice experience with Grub2 is that after manually editing grub.cfg an updater must be run to incorporate the changes. Since I can only get into the Windows PE environment on that pendrive I have no clue how to update grub. I've tried messing with Easy BCD but I'm even less adept at Windows work than Grub2.

All of the guides I find online are for Ubuntu and the like employing some repository to apt-get or whatever package management to configure Grub2 on a portable drive.

Naturally I'd rather do this in Slackware. Can any of you Grub devotees help?

All of the guides I find online are for Ubuntu and the like employing some repository to apt-get or whatever

This is because debian based distros separate grub2 into several smaller packages for efi or legacy booting. Slackware doesn't do this so no additional packages to install. With Slacware the basic install command will be something like this for legacy:

grub-install --target=i386-pc --boot-directory=xxxxx /dev/<usb drive>

and for uefi

grub-install --target=x86_64-efi --boot-directory=xxxxx --removable

--boot-directory will be <usb mount point>/boot
For uefi mode the usb will need a fat32 partition mounted at <usb mount point>/boot/efi
before running grub-install

I've partially succeeded by installing Grub2 to a test USB drive which will boot in both modes but I'm failing to get a menu for selecting the 2 operating systems on this pendrive.

A grub.cfg file without format/syntax errors in the correct location will give a grub menu.
At the grub prompt

grub> set

will output information about the grub installation. The prefix line will display where grub points to for the grub.cfg file.

My totally novice experience with Grub2 is that after manually editing grub.cfg an updater must be run to incorporate the changes

Assuming you are talking about grub-mkconfig/update-grub. No you do not need to run an updater after editing a grub.cfg. Running an updater will overwrite any edits made to a grub.cfg.

Do you want to do this as an exercise or do you need it to work?
If it's the latter, maybe there's no need to reinvent the wheel and just use Ventoy?

As above, Ventoy will do what you want.

https://www.ventoy.net/en/index.html

Thanks all. Sorry I missed your responses for so long. The consensus seems to suggest at least trying Ventoy so I will happily do exactly that. Looks good!