Converting variable voltage to constant voltage / variable current
The adafruit module looks good but it's only really meant for input voltages below 6V. To maintain efficiency you should use a low drop-out switching buck regulator capable of withstanding the 30Vs you might apply.

I'd recommend the AD8610 (mainly because I've used it on two designs): -



It's maximum input voltage is 42V so be aware of this. On one design I put a switch-off-when-the-volts-gets-over-35V circuit with a couple of MOSFETs just to protect it.

When you under feed it with voltage it still pretty much acts as a voltage follower be be aware of the under-voltage lock-out it needs to be tied to Vin to get the most out of it.