Is it possible to flash multiple ISOs on one USB?
Google for Ventoy GUI
https://ventoy.net/en/index.html
https://github.com/ventoy/Ventoy
Howto Install Ventoy to Ubuntu examples: version 1.0.43
https://x8t4.com/how-to-install-ventoy-on-ubuntu/
Other URL links:
https://9to5linux.com/multiboot-usb-creator-ventoy-adds-a-gui-mode-to-its-live-iso-image
https://www.linuxuprising.com/2021/09/bootable-usb-creator-ventoy-gets-native.html
https://linuxiac.com/ventoy-create-bootable-usb/
https://www.libhunt.com/compare-Ventoy-vs-rufus
https://levdev.blog/2021/09/ Ventoy and Etcher.io
Good luck with the multiple distros of your choice
ps Here is my puppylinux blog about Ventoy with a Python GUI wrapper.
https://puppylinux-or-pcbsd.blogspot.com/2020/12/venpy-gui-frontend-to-ventoy-multiple.html
My distro choices, GhostBSD.org Puppylinux.com https://manjaro.org/
Correct, Ventoy is the way to go for this particular use-case. 