If viewfinder darkness is a major concern, you can use an auxiliary focus assist light, which I find to be especially helpful at higher mags. It isn't bright enough to affect the photos at ISO 100-200, even with my very high-intensity 3-watt/80 lumen LED flashlight. I use 2 rechargeable Eneloop AA NiMH batteries, good for at least 1 hour of continuous use in this flashlight.
I find that most bugs do not react much to this very high-intensity light, which I hold in my left hand pressed against the lens barrel. They instead mostly react to dark shadows. If anything, they seem to become briefly paralyzed by the flashlight. This helps you see the finest detail in the viewfinder.
I hope this option helps. I gather that most of the best macro photographers don't use a focus assist light routinely, but perhaps it's worth trying as an ordinary mortal. They cost 25-50 US dollars online with these specs if you shop around - no need for the very expensive police/military flashlight models. The inexpensive Chinese models work fine, or at least mine does - it's by Mallory, purchased at Wal-Mart. Having at least 80 lumens output (3 watts) is helpful.