[bazookajoe]

This is what I tried like Cascade94 said:

adb devices = fine, device is present
adb reboot fastboot = produces that menu picture I uploaded on my screen. I've tried this 10 times at minimum. Same result every time.
fastboot devices = to check if fastboot is working and device read (yes it does)
fastboot reboot bootloader = appears to work get an OK message. Screen turns to ONYX hooray.
fastboot boot mypatched.img

<waiting for any device>.

I've also tried going directly to bootloader:

adb devices
adb reboot bootloader
fastboot boot mypatched.img

<waiting for any device>

I've tried no bootloader at all:

adb devices
adb reboot fastboot
fastboot devices
fastboot boot mypatched.img

That just gives an error because it doesn't recognise "boot" because bootloader and I have boot_a and boot_b blah blah.

I'm not really willing to try fastboot flash boot_a mypatched.img, if I can't even get it to boot to check if the .img works. I just won't. I've always booted up to see if root worked in Magisk before properly flashing/installing. Always.

Maybe there's a special syntax that I'm not understanding because of two boot partitions but still, Cascade94 apparently gets their images to boot to test and see if it works unless things have changed since their last guide.

I understand I could flash through edl but I'd really rather work out fastboot first/understand my device properly.

Edit: and how to boot a patched image first before flashing. That's always been an option available I want to understand how that works on a device with two boot partitions.