That setup could draw 300-500mA from the battery when GPS and WiFi is active
(6-8hours before a fully charged batt is flat).
With that current, every 2-3ohm of the pot drops 1volt.
I doubt that you can adjust a 10k pot to <50ohm, so a 10k pot is totally unsuitable.
Why D2. It's only function is to make the boost converter work harder (and use more power).
A Mega is a poor choice for a solar/battery project.
Are you sure you can power those two devices from the pins of a Mega?
Most boost converters are not happy with a 10000uF capacitive load.
A 2watt solar panel might deliver 2watt (0.33A) in full sun.
Only ~1.4watt (4.2*0.33) of that could end up in the battery.
You might charge that battery in two full sunny days (if nothing is used).
Leo..