I don’t know what you mean by “within the ISO-OS”
I refer to this thread: There it is mentioned “… A StandaloneVM is similar with other VMs you may have used outside Qubes OS…” and from the further explanations on them main documentation about Standalone/HVM, I got that both do not support AppVMs on their basis (this is possible only with Template VMs).
But you are right - EndeavourOS is based on Arch as well as KDE neon is based on Ubuntu. However they have their individual flavor which makes them more convenient than the “originals”. Best example was my problem with Manjaro: It worked perfect for about 3 years with my brother MFC-J6520DW but suddenly (after an update) it could not print anymore in duplex-mode whereas EndeavourOS does without any problem. Both distros keep KDE versions up to date, whereas the “originals” stick on the (long term) stable repos.