Phone stuck in fastboot mode
The problem was that I edited the system hosts file. This causes the system to be unable to boot; the reason is dm-verity which denies boot (chain of trust) when any modified system file is detected.
To fix the issue, I followed https://www.droidwin.com/restore-oneplus-6-stock-via-fastboot-commands/
Basically, it explains how to download the stock ROM, extract the .bin file into multiple .img files, and how to flash them subsequently.
Since only the boot and system partitions were modified, and I did not want to lose my data, I flashed them using
fastboot flash boot_a boot.img 
fastboot flash boot_b boot.img 
fastboot flash system_a system.img 
fastboot flash system_b system.img

This fixed the issue =).