Wanted to share with anyone else who was struggling with this combination. I wanted to get 4k video from the iPhone fed directly in to OBS on my MacBook connected via USB-C. Everything seemed fine, but I couldn't get OBS to show Telecast as a video source, finally realised it had to do with macOS Sonoma removing legacy video support. Wanted to share the solution with others who might have been looking to do the same.
Getting 4k with Telecast and iPhone into OBS
Getting 4k with Telecast and iPhone into OBS