I agree that flathub is like killing flies with cannons.
May be a good solution when the base OS and environment is too different from the environment that is needed by the target program, and it is a complex program with many dependencies.
For example to run an obsolete program that does not run on a modern environment and it depens on many deprecated libraries.
But for general use it generates too much overburden.
Appimage seems a better way, more light and still seems to provide isolation and distribution of needed dependencies.
The only draw back is that it does not provide an installation or updating process nor integration with the graphics environment (for example links in the application menu) that has to be made by hand, but not a difficult task.