You might find this thread interesting: Additional external camera - #18 by Skredfare
I’ve mounted a GoPro (7 and 8) in a 4’’ tube, where I convert the signal from HDMI to NDI and send this to my topside computer or separate screen (basically any NDI-capable viewer connected to the same network will do).
The GoPro Hero 8 will give more or less delay free video via the network on NDI. The Hero 7 gives 2-3 seconds of delay when stabilisation is enabled.
The software used is https://dicaffeine.com/ and the hardware is a Raspberry Pi 4, with a HDMI-USB video card, in addition to a separate battery. It all fits in a 4’’ tube.