Thanks again for your reply, I really appreciate it. I downloaded the firmware and the flash tool, but unfortunately it didn't work
I put my phone into fastboot and pressed refresh on the flash tool. My screen went black and showed the tiny 'Press any key to shut down' text at the top again. The flash tool kept running but with an error (see attachment). My phone is definitely unlocked. The weird thing is I even did a factory reset, out of desperation, within whatever built in recovery this is, but CrDroid is still booting as if nothing happened.
So basically I have CrDroid installed, with a random built in recovery, no gapps/playstore and no TWRP. I'm close to tears with frustration at this point.