Hi everyone
I discovered Ventoy as an alternative to YUMI for creating multiboot USB sticks and it works well ! As a short summary, you drop the ISOs in the designated partition and voilà, next reboot you can boot them. There's more details on the official website on which ISOs are compatible or not.
On a Acer Aspire with a Insyde Corp BIOS, it's possible that although you disabled Secure Boot in your BIOS and USB you still cannot boot the USB.
So, what you should do is plug in your USB, reboot, get in your BIOS and navigate to the "Security" tab and below "Secure Boot Mode" there'll be an entry labelled "Select an UEFI file as trusted for executing".
If you navigate to it and Enter, you should be in a bare-bones file manager. Find the file ventoyboot.efi (it's in the EFI/ folder in most cases) and select it. Reboot, press F12 to bring up the boot manager and select ventoyboot.efi: you should have the Ventoy GRUB-like menu appearing with your ISO filenames listed.
ventoy -u -S /dev/sdX # this command is non-destructive, there's should be no data loss on your USB key