When I go into “Library > Font Styles > Metronome Text Font” then I can change the font weight from Regular to Bold. However, there doesn’t seem to be a way to make the note thicker. In addition, when I go into “Engraving Options > Notes” then I can only change the stem thickness in the music itself.
My printer isn’t that great, so I’d prefer the thicker stem when possible.
I think you are out of luck. Unless you can find a SMuFL music font that has thicker stems.
Further to what Andro has posted (while I was composing my reply):
If you can put up with the notehead also being bigger, go into Library > Font Styles… > Metronome Music Text Font and increase the point size. Of course, you will need to experiment a little to find the appearance which displeases you the least. You will also be able to change the font used, although it needs to be SMuFL. As each font family has its own look, you might find one which has a thicker stem without the need to increase the overall size of the note used in the metronome marking.
I tried about six SMuFL fonts I have, to no avail.
Perhaps this could be a feature request.
It’s an edge case, but I came up with a work-around this morning. I type a “|” as text, and then I put it behind the note. Works for the time being. Thanks for your help everyone.