I don't want to take it on a weekday because I have work and no paid vacation, so taking a day off would cost several hundred dollars of income. As unpleasant as it seems, it's cheaper (time and money wise) to take the 2 hour trip to Sacramento.
However, right now I'm inclined to believe that the test centers are not actually fully booked, it's just a technical glitch that's showing most of them as fully booked. If I search for tests 5-6 months in the future, the system still reports them as almost fully booked; this is absurd, so my results are almost certainly because of the glitch. That being said, the system glitch has persisted for 3 days now, and I don't have any information about the true availability of the testing centers, not even by calling them. It's really unacceptable that a function central to the system, registering for the GRE, remains completely broken for 3 days, but hey, what can you do.
For now, I'll have to wait for the system to fix itself before committing to a suboptimal test date / location.