[Matt Devo]

I do not remember seeing the option to plug in a flash drive to create the LibreELEC install media.

it's the menu option directly below the standalone firmware, it must be manually selected.

And when I rebooted the box, it now boots to a black screen. Same for trying developer mode.

the standalone firmware boots directly to SeaBIOS, there is no more Developer Mode or Recovery Mode. Since ChromeOS isn't bootable from Legacy Mode, it just tried and fails.

If I plug in my recovery media, it will loop between the chrome screen, SEABIOS screen, and a black screen. But it doesn't appear to have any options to actually recover, and no developer screen ever appears for me to ctrl+d to go to Chrome OS.

Again, I know this is my fault for taking the boot options part as optional, but I would appreciate any help..

see above - there is no more Recovery Mode (so the Recovery media won't work) nor Developer Mode (so the CTRL+D/L keys have no function).

all you need to do now is simple - boot a USB and install an OS. From another machine, download the LibreELEC USB/SD creator tool, and use it to download the LibreELEC install image (Generic x86_64, v7.90.007) and write to USB/SD. Instructions are on their site, but it's pretty straightforward.

after creating the install media, insert into the Chromebox, power on, and press ESC when prompted to select and boot it. Run the installer, install to the SSD, then when prompted to remove the install media and reboot, simply press enter, then power off via the power button (their current has an installer that causes it to hang at the end). Remove install media, power on, and it should boot right into LibreELEC in about 5s.