Does this solar charger PCB design satisfy the layout requirements of the step down charger?
I like the C2 and C4 sharing a common GND spot. Go ahead and drop a via, right there, down to your GND plane.

And add couple Vias from D5 down to the plane.

I view every VIA as 1nH or 2nH, depending on periphery/height ratio. However the current crowding into the VIA, and current crowding as exits the VIA, and broken-up traces and planes, will erase the benefit of extra VIAs.