I little bit offtopic. Look at the Ventoy project. It let you create multiboot usb. After installing it on usb drive you simply copy isos on it as usual files. Any amount depending on usb size. When you boot from it you’ll get menu and can choose iso to boot. Even you can boot iso from another drive.
That looks fantastic — thanks for mentioning it @borispr 
Just need to find a big enough USB flash drive.