Zblerg, I’m always confused about the “boot flags” and so on … From my experience, I don’t trust unetbootin because it has too many options and I’m never sure it sets the boot flag properly, whereas I trust Etcher on this … (though granted it’s a bloated piece of electron software, 100+MB just to flash an sd card, wtf…)
Alternatively, you can try to use Raspberry’s official flashing tool (Raspberry Pi Imager) : https://www.raspberrypi.org/downloads/
and select a custom image/file in the list before the flash procedure