I recently did something similar but as a Mac app.

It sounds like a similar stack, but distributed as an app. FFmpeg (LGPL compilation).

I haven't tried Pixi.js, looks interesting. I guess it was good for this.

Have you looked at remotion? I found them good for somethings, but ended up using Safari for rendering (instead of remotion's chrome-based rendering) because app packaging was easier that way.

https://www.loremlabs.com/cliproom if you're interested in comparing