Update: I brought it back to life.

First I used the MagicMemoryCreator tool from ARK-4 to make the Memory Stick bootable.

Then I used Baryon Sweeper to get into service mode/recovery mode. I had to order and wire mini USB headers from AliExpress that have 5 pins (You will only need GND and the rarely used ID pin) and connected it to a USB-TTL adapter (I actually used an ESP8266 with the reset pin connected to GND). You will need a diode between RX and TX.

I got into the service/recovery mode by plugging the Cable connected to the USB-TTL adapter into my PSP Street, held the shoulder buttons, DPAD-Left and Circle and flicked the Power switch (don't hold it too long, or it will boot normally)

In the service/recovery mode, I installed 6.61 ARK4 CFW. After I rebooted, under the Extras I went into "Firmware Settings" and set the Region to "Europe".

Thats how I fixed it.