Phone bricked after reboot (Fastboot stuck in 'Device-State: error')

Yesterday, my phone was acting kinda sluggish, so I rebooted it… And it never came back up.
I force booted into fastboot and was greeted with this [screenshot below]

Now, booting to Recovery and Rescue from the bootloader just goes to black screen (Start goes back to the bootloader), and booting normally also just goes to black screen.
Trying to do most things via fastboot cli just results in the error message FAILED (remote: 'error getting device locked state Protocol...

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Source: XDA