To answer my own question…
In the “settings” pop-up dialogue, there is a little tab to the far right of the other tabs (filters, multiplexer, video, audio, etc.).
If you click on that little tabb with the four lines and the triangle, one of the options is “use maximum render quality.”
Why they hide this option here instead of making it a simple box to tick, I have no idea.
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This is my life, I edit and edit and edit and edit…