Weird, worked pretty out of the box here besides some of the v4l2 parameters for application compatibility. My system is a bit outdated though, v4l2loopback-dkms 0.9.1-4ubuntu0.1 and OBS 24.0.3 (64 bit, installed through whatever the quickstart guide suggested for Ubuntu).

I'm using this to initialize the loopback device:

    #!/bin/bash
    set -e
    if lsmod | grep "v4l2loopback" &> /dev/null ; then
        echo "v4l2loopback is loaded"
        lsmod | grep "v4l2loopback"
        sudo rmmod v4l2loopback
    fi
    sudo modprobe v4l2loopback devices=1 video_nr=2 exclusive_caps=1
    lsmod | grep "v4l2loopback"