I haven't used swap on zvol myself, but I've read about issues in low-memory situations, like this one: https://github.com/openzfs/zfs/issues/7734
To write to swap, zfs needs to allocate ARC memory. But to do that, it would need to write to swap...
Consider using zramswap, that's probably the easiest solution.
As far as I know flatpak applications can be distributed as a file without the need for a repository, just like .deb or .rpm files