Here are some projects that I am or was involved in :
I maintain the Discord Flatpak but I've recently started working on updating applications from EOL runtimes all over the place.
This is a tool I made to help myself find problems before I try to make a pull request and see it fail, I've made fp-build to have things like checking for 128x128 icons, check that appstream is valid amongst other things.
These are some python scripts to generate data about Flathub applications such as what architectures they support, what runtime they rely on. I want to expand it to check for other types of data that we might want to improve upon.