The video widgets don't/won't matter. gstreamer doesn't build the gtk4 widgets by default because it queries everything up front which pulls in the libraries so they can't really provide gtk3 and gtk4 sinks at the same time and right now they've opted to continue support with the gtk3 widgets.
I tried finding the blog post about just rendering a widget right from the pipeline but couldn't find it. Either way, that's a problem for another day.

Ship It!

Great work, thanks!!