DDG ends up working well as the default search for me. The ability to type my query and then decide if I want to use DDG or Google doesn't require me to take my hands of the keyboard. I just add a g! and I'm switched off my default. So it's a great default with an ergonomic way to switch to a different engine.

Google as default requires me to use the browser UX to change the engine, which I believe requires using the mouse or hitting tab or arrow keys. Less ergonomic.

Firefox supports this, go into Settings -> Search and set a keyword for the search engine.

For example, if you set the keyword for DuckDuckGo to "ddg" and Google to "go", then typing "ddg <your query>" into the topbar will search DuckDuckGo, and typing "go <your query>" will search Google.

I've set Firefox its default search engine to DDG. Then if the search results are unsatisfying, I search again but now prepend the search with !g for Google, or !gi for Google Images.