Update - upon finding others with similar issues, I thought I’d try a different I2C display. The one listed here Issue trying to connect I2C OLED display to Pixhawk - #10 by Erik_Groeneveld and here Issue trying to connect I2C OLED display to Pixhawk - #17 by rob215x both worked for me back on the non-custom firmware flashed to this board while only having to change the NTF_DISPLAY_TYPE parameter, as I had originally tried. No custom firmware was needed, just the right hardware! I don’t know enough about I2C to know why certain displays work and others do not, but it seems it might have something to do with the I2C “address” - from what I could gather that others were mentioning.