Yad is fantastic! Write the core of your app in bash/curl/jq, then write a thin wrapper to invoke yad so your app is presentable to non-techie people.

Also see:

https://wiki.gnome.org/Projects/Zenity (also gtk, less features, GNOME project)

In KDE/Qt domain:

https://github.com/KDE/kdialog

https://github.com/luebking/qarma