Yay for Inkscape, Scribus and everything Libre Graphics! One of our main workhorses is Inkscape and am sure my partner would find it much easier to participate here.
As for video, I had something like this in mind a while back. I would suggest not to divide by programs but use a general video editing category. Shotcut has a private forum and Kdenlive uses the KDE one. I would enjoy a mix of r/editors with r/Linux_Filmmaking with a pinch of r/videography.
Great initiative! 