Solar power for your shed/garage/cabin (smaller than a house, basically)

If so, I'm intending on interrupting that circuit from the smart battery protect to the input terminal on the inverter with a Shelly1 WiFi switch, which will give me the ability to turn the inverter on and off via the Shelly app on my phone.

Yes, that should be fine.

The Victron diagram shows a small inline fuse to prevent a short circuit on that skinny wire making it catch fire. You could move your wire to the far side of your mini fuse block to achieve the same thing.

I think my system needs more panel

It's only going to get worse as winter goes on. Keeping that wifi router going 24/7 on a week of January gloom seems ambitious.