I'm trying to install GrapheneOS on a Pixel 6 Pro (yes, I know) from a Pixel 7a that's already running GrapheneOS. When I tap the "unlock bootloader" button in the web installer, "no compatible device found".

The source device (Pixel 7a}…

… is in the owner profile
… is running the latest stable version and Vanadium
… does not have the web installer running in an incognito tab
… does not have developer options enabled
… is not rooted
… is running Vanadium with the default settings ("clear storage" was performed on Vanadium's app info page to be sure)

The target device (Pixel 6 Pro)…

… is currently running PixelOS
… has OEM unlocking enabled in the developer options
… does not have any additional developer options changed
… is not rooted
… has been rebooted into fastboot

Both devices…

… are running the latest system update from the stable branch
… are connected to each other with the original white USB-C cable that came with the source device

I've also tried changing "USB controlled by" to "this device" on the source device; it makes the attempt but eventually fails, saying it "couldn't switch".

What else needs to be done? I'd rather not use my laptop, it doesn't run one of the officially supported operating systems required for facilitating the installation. It's not even in the same family tree as any of the supported OSes, so I'm reluctant to just wing it and see what happens. I want to do this "by the book".