The above link describes how to have a rescue ISO at hand simply by adding it grub
Nah - that would require some extra work - but it is doable - before ventoy there was a project which made it possible to add ISO and they would be detected.
I wrote a guide long time ago - I think it is doable - [root tip] [How To] Multi ISO USB with storage partition
The original project referenced in the topic is https://mbusb.aguslr.com/ and the sources is at GitHub - aguslr/multibootusb: A collection of GRUB files and scripts that will allow you to create a pendrive capable of booting different ISO files - it is no longer maintained but that doesn’t imply it is dysfunctional.
As I recall it is crazy simple to use - it works much like ventoy