Pablo Rodriguez schrieb am 02.09.2023 um 12:35:
Sorry, but I don‘t know why this doesn‘t get the slanted font as bold (I
know I should know better, but I‘m stuck at this):

  \definefontfamily[mainface][rm][DejaVu Sans]

You're using the wrong symbol (you have a math operator in your example) to assign a value to a key.