TBH it’d be nice if Lemmy just grabbed a random part of the video and made a thumbnail out of it

I mean, ffs if Lemmy could at least just grab the og:image and og:title metadata that the site themselves share, in the case of sites like YouTube. That wouldn't help with your own hosting site which doesn't have that sort of metadata, but it'd make the experience a lot better for the larger number of people sharing from YouTube, where they get no thumbnail and have to manually choose a title, when any other site generates the thumbnail and suggests a title automatically.