I finally got to test the video synth with a VGA signal generator, following this tutorial, and the results were far more pleasing..
I also realized that I had reasonable affordance with the voltage, and voltage fluctuations lead to interesting visuals :
Voltages between 5.5 and 12 seemed to work. I had initially been under the impression that I would need at least 7V, and was gearing up to use a dc-dc booster, but it seemed that perhaps directly using the fluctuating voltage from the solar panel could be a good option..
I tested the current draw and was pleasantly surprised to see how low it was :
I forgot to take a picture without the battery charger, but it was about the same voltage (5.5V), with more fluctuation.. but for some reason the circuit wouldn't power with it. I need to investigate this more, on a sunny day..
I also played around with the dc-dc booster (though I probably won't end up needing it)
Next steps would be to try directly powering with the solar panel again, triple-checking everything, and in more direct/stronger sunlight.. If that still fails I could use the dc-dc booster, though I hope it mirrors the fluctuations with the input. (otherwise it has a tiny screw for adjusting how much it should boost to)