If you want to help with code, have a look at the issues page at GitHub. Aside of open bugs there are also ideas for new features available.
If Nagstamon is still missing in your favorite Linux distribution or outdated you might help out there. Especially the Flatpak is extremly dusty. It maybe could even use a new start without the ifw_dresden name.
The web pages could need some finish, especially in the documentation area. With GitHub Actions now being used to create GitHub Pages it is now an option to switch to another Markdown-to-HTML renderer like Hugo.
To help out with paying code signing, domain and mail, a coffee or donate in general just use PayPal directly or send to firstname.lastname@example.org.