Open core never seems to flash a Boot Loader onto my USB installer

This is OpenCore 2.4.0 - I’ve been trying to build a USB installer of Sequoia on an old i7 core duo iMac, which is currently running dosDude Catalina (but I never get to point of install on that anyway) (will follow up with more specifics, but…) Am I missing some obvious step? Build of installer onto the USB runs without a hitch, then: Im presented with a UFEI icon in the picker, but this takes me only to predicting (dosDude ) install…