Following up here, I have now tested this with three different dongles and can rule out, with relative confidence, that the issue isn't dongle related.
Thankfully, I have managed to nonetheless solve this issue to some extent so hopefully this will help others facing similar problems in the future. Two main things you need to make sure when setting up the connection with the dongle:
In network settings, not only do you need to make sure you "Manually" specify the IP address to something within the 169.254.XXX.XXX range with a 255.255.0.0 subnet, but you need to also make sure that the Router value is set to the same. So for example, for me, this looks like:
You need to make sure that the Dongle you are using to connect to the PoE camera is the first connection in the "Service Order" within MacOS settings. This is the only way I have found for it to work reliably. In my experience, if you don't ensure this order is kept, you might be able to connect to the bootloader on occasion, but even if you succeed, you'll generally get Failed to find device after booting, error message: X_LINK_DEVICE_NOT_FOUND.
To view and reorder the Service Order, click on the ellipsis button at the bottom of the settings and reorder the entries until your dongle comes out on top:
NOTE: When you reorder the Service Order in this way, you will lose access to the internet until you place an actual internet connection back above the dongle.
This is how I have gotten to work, if anybody digs into this any further and figures out how to get this to work without having to simultaneously lose access to the internet, I'd be grateful if you could share back.
