It looks like you’re not using the desktop-launch script from https://github.com/ubuntu/snapcraft-desktop-helpers, so you might have missed some of the things that it sets up for you.
Generally, for desktop applications I recommend that either the gnome-3-28 extension be used or the Snapcraft Desktop Helpers repository is integrated to a Snap’s yaml.
There’s a helpful document explaining how to integrate for qt5 at [Deprecated] Desktop App support - Qt5