well, meanwhile, I gave up completely, switched to Obsidian (where iFrame embedding works out-of-the-box - iFrame sources may even be one's own note attachments: don't tell me anything about "security concerns"!), found a way to synchronize notes between devices for free within minutes ("for free" is important for my students) which means: I got everything I looked for.
I like the idea behind Joplin, and it seems pretty well done - but without iFrame embedding, it's just a better text editor.