@Araminta Thanks for the confirmation. Yes, please do look at this when you can. It could be improved for sure.
For anyone facing the same problem, and needing a quick fix for video overlays, Lity seems to work flawlessly with Cwicly🙌. All you need to do is add the Lity CSS as a global style sheet, and the JS to <body> end in Settings > Custom code. You can then simply link to the video URL and add data-lity as an attribute… it then just works. Bonus, focus returns to the correct position on exit too, unlike the Cwicly lightbox.