It would be great if the frontend and backend were separated with a unified API and you could simply choose a frontend/interface (Vivaldi) with whatever backend/engine (Gecko). That’s not how it (currently) works though.

Arc has floated this idea. Currently Arc is Chromium-based, but they say they’ve designed it to allow for swapping engines in the future.

IIRC, Edge had a similar feature for a while, allowing you to run legacy Internet Explorer tabs if a site required it. Not sure if that still exists.