I love .appimages they work on all distros, easy to install, works from any directory, works without installing
If I can achive this setup it will make switching my work from machine to machine easy-fast, resilient, and distro hopping wouldn’t be a issue.
I see two options.
The simpler of which is to have a wrapper script that says HOME=/custom/path/for/appimage. Apps that correctly follow xdg-specs will then put all their data in that path. But not all apps will. Apps that put stuff in /home/$USER will not use the correct location.
The more foolproof way would be using something like bubblewrap, which is used by flatpak. With bubblewrap, the sandboxing can make /home/$USER appear as /custom/path/for/appimage. However, this would take more work to setup, since I presume you want the appimages to feel unsandboxed.