Lucas, don't get me started on the folly of GPSs again
For coastal sailing, I posit there are very few (or make that limited) use cases for a GPS to determine location. Most of the time, navigation by landmarks works just as well, or better, for your purpose. Even if you need the GPS to tell you where the next waypoint is, as you discovered, you are usually best served letting the wind determine which angles to sail. You can count yourself lucky having learned that early on.
In light wind sailing, the goal is to sail to where the wind is strongest and stay with the strongest wind as long as possible. That usually beats trying to sail the shortest distance (within limits). People racing sailboats hone these skills, but when you are trying to make it back to port, in a situation as you described, you are in a race against time, effectively. Might look into some books on small boat racing over the winter to pick up some tips (or join your local "beer can" race, if there is one in your area).
Besides hunting for wind, the challenge in light wind sailing is to trim your sails, because you will not necessarily get the same degree of feedback as you are used to with brisker winds (tell tales will be limp, etc). In really light winds, depart from the usual attempt to sail the boat flat and heel it to leeward. The weight of the sail will then help create the sail shape, and you reduce the wet surface (and its drag, which matters more when boat moves slowly). Finally, it is said, that sailing in weak winds tests your conviction. In those conditions, rapid change of plans (as indeed any rapid movement within the boat) is deadly. It kills what little momentum you had managed to build up.
I assume you know about letting out the outhaul a bit, and not tightening the halyards too much when the wind slackens a bit. Except, in very light winds, were a flatter sail shape is needed again, or the airflow won't manage to stay "attached" to your sail. For the longest time I've been unable to pin down people on what the precise cut-offs are for these wind speed regimens. Now, around 6 knots of wind, the wind enters turbulent flow; you will notice that it feels much stronger than 5 knots, despite there being only a small change in speed. My best understanding is that light wind starts from that transition on down, and very light winds are in the "barely any wind" realm. (Inviting others here to complete the story).
If it's any consolation, with some experience, you'll probably get faster boat speeds out of marginal winds. I set out on an "ice-cream" run once, that famously took 6 hours one way, 9 hours total and had us return after midnight. We've done the same several times now, but we've always ended up faster; we still get home at midnight a lot, but now we are having dinner at the other end, not just ice cream.
Good thinking on breaking off your trip to recover. Gear suggestion: get yourself a 9# Danforth anchor. Even if you don't leave the boat at anchor, you can hold it off the beach just a bit, so it won't get banged up next time.
Good luck next time!