Librewolf supports letterboxing as well, though the setting might be disabled by default

Agreed. That’s a really bold claim to make without citing (credible) sources

I’m running kubuntu on your same 2012 MBP. Runs great, but I’ve considered going pure Debian too, maybe still with KDE Plasma. You look happy; cool background!

Well done!

Just a heads up though, there’s an existing server/client app combo also named Alexandria commonly used in public schools and libraries (in the US anyway). It’s more of a book inventory and check-out system, but the shared name for a related app could still cause some confusion, and might prevent schools and libraries from utilizing your helpful tool.

Just thought you should know. Your efforts deserve to stand on their own!

+1 for kdenlive. you could also try OpenShot Video Editor or ShotCut, but I prefer kdenlive