I have an N2 (and N2+) with the latest petitboot spi image that HK published and all I do is write the SD card (or eMMC module) and ensure the boot switch is to the right, and it boots into LE without issues. I can't say I've ever tried USB, but mainline u-boot should support that too.
By saying newest SPI from HK you mean spiboot-20200511.img?
Your images is working, but only from SD card and with switch on the right position, which seems to be different form balbes150 images. There is no more screen flickering, so thanks for that.
You may be interested that both your and balbes150 images have bug (or LibreELEC does), that if you start Odroid with no kodi configuration (second partition from the image) then if you change language on first screen (in my case to Polish) then everything freezes and you have to purge second partition to make it work again.