Alright, so I’m an idiot. I knew secure boot needs to be disabled, but I couldn’t find it anywhere, so I thought it’s disabled by default. Turns out the BIOS is so crappy on this computer that you can only access advanced features (such as secure boot) via hidden shortcut. No option to enter Advanced mode directly from BIOS itself.
In case someone needs it in the future: once in the BIOS, click CTRL+ALT+F7, and hidden settings will appear (works until reboot). Installing Manjaro right now!