The US military would disagree that all drones are toys. Not to mention all the industrial uses. Some drones are professional tools, some are toys, some are beyond toys but used for pleasure IMO.
Hi, welcome to FPV! I have a few thoughts on my preferred hardware:
FC: F3 based SP Racing 6d0f, or KISS
PDB with OSD: Matek HUBOSD
ESCs: basically any 30A BLHeli_S
Motors: 2205 or 2206 with 2300kv+
FW: betaflight (stock PIDs just work)
VTX: Tramp HV (a bit costly but worth it)
Cam: one...
It appears that the DJI batteries include an IC which may internally drop the voltage sent to the cells during chrging? It looks this way to me because the batteries have an on/off switch, LEDs, diagnostic indication, etc. Just an educated guess, as I am not familiar with "buck converters".
A typical lipo cell has a nominal voltage of 3.7V and charges to 4.2V. They should not be discharged below 3.2V (3.0V might be ok). I don't know anything about those proprietary DJI packs, but a normal 4S lipo battery should stop charging at 16.8V. Are the DJI ones really 17.5V?
Sounds like it has to be the camera, not the vtx or the goggles, since the camera settings OSD is showing up in the goggles. My guess would be that 7.4V may be too high for the camera. You might try to put a 5V BEC between the battery and camera. Does the camera give an input voltage range...
So we know the camera is sending the signal, and the vtx is working since the OSD shows up in the goggles. Silly question, did you remove the lens cap? I ask because I have missed this before. You should get snow, not black screen on those goggles if it isn't working.