@business_kid
I read your post again, in depth, and realized i don´t quite understand it, which to me means it may contain a good answer, so I am willing to try it but would like to check with you before I start:

my /dev/sdX1 where i have my folder /ISOs at root level, with my isos is formatted fat32, the isos are (I suppose) iso9660. Was this the point you raised ?

GRUB does have an "insmod 9660" and I have succeeded in using multisusbs done this way in several different computer for a couple years.

The greatest problem once Grub menu is up is the right text to the grub stanzas. I have been able to many times solv that problem. Sometimes by looking into the stanzas in the isos themselves and eventually adapting them, since sometimes they are at isolinux and it is not grub format. Sometimes I edit and re-edit a stanza many many times, within the Grub menu before it gets to boot its distro. Often the problem is some silly typo. But there cases of conceptual inadequacies.

Some stanzas i have found in forums for specific distros. Sometimes I extract the iso into a sub folder of /ISOs and them i have success in writting a stanza that works.

If it should interest you I could send my grub.cfg and mark the ones that boot - not all at all.

Again in my case I think that EFI has not been a problem, Since none of my computers use it. But again, I may be wrong.

Read and act on post #8, and/or post #9. I know what I'm talking about, but I'm woolly at times. My point was that you're asking an awful lot of grub, in a word. And that grub may not be up to it.

Or that "this may not be the grub you are looking for". Check int the documentation about how the suggested tools E2B and Ventoy (and others) make it work. They do not generally use stock grub, or not it alone. There are clues, but if it was easy everyone would be doing it.

Ventoy is now available in the Arch repos.

I feel inclined to try Ventoy first, men right now I got involved in another project, so it will be probably a while. I will report on how it goes.

Thanks again for your help.

I have found what i am considering a good solution - so far : I full instaled the base of ArchLinux to one of the usb sticks I was having problems to boot in 2 of my computers. Installed grub to the MBR.

For some - to me - strange reason the usb does not bot Arch Linux. BUT IT GIVES me GRUB on the screen, although in form of a GRUB boot prompt. BUt that was all I needed. I copied the grub.cfg from one of my multiboot-usb-sticks to it and - Voilá! - it gives my menu when i boot on the Fujitsu Lifebook.

Now I will start copying all the isos and when i give up trying to boot those i still can't boot when I get tired of trying I will look into why is Arch Linux not booting. This was was the first time i attempted to install Arch Linux.I kind of liked it. So thanks you all, This is SOLVED!