I’m aware I can select note heads, r-click, and select, say, Notehead | Diamonds | Wide Diamonds.
How would I make that the default notehead set for all notes going forward? (I’ve read the documentation/viewed the videos on notehead sets, and I’m still a little fuzzy about this!)
You can add the noteheads to a selection of notes (CMD/CTRL+Click non-continuous notes or SHIFT +Click contiguous notes) and then Right-Click on one of them to assign an alternate notehead to the entire group.