It looks like it may be possible to create multiboot partitions on a USB stick.
Look at https://www.ubackup.com/windows-10/multiboot-usb-windows-7-and-8-and-10-rufus-0025-rc.html
An alternate option is https://www.geeksforgeeks.org/create-a-multi-iso-bootable-usb-drive/
Before I read this, my thoughts were to create as many partitions as you would like on the USB drive, each with a different drive letter of course. Then, choose one of the partitions for the ISO file or the like installer, you wish to create. Continue until all partitions are filled or you created as many installers you need. Never tried this, but I would like to know if this works for you.
Justin