For those following my journey, I got the MBP in to DFU mode, restored Sequoia, turned off secure boot, downloaded the latest Omarchy 3.0.1 offline ISO, booted and installed Omarchy. No external keyboard needed, no Ethernet needed. Way easier than 2 weeks ago.

A laptop screen displays a vibrant sunset landscape with mountains and a deer, while system notification pop-ups appear in the top right corner.