However, I was wondering if this could be some sort of fungus?

I started this lettuce on March 1st. Then at the beginning of the week, I started to notice the outer leaves getting small brown spots, translucent/transparent spots, and some minor yellowing. As I pulled off the outer leaves, the next healthy inner layer would have the same issue when I woke up the next day. It wasn't until Tuesday that I saw the spider mites, but I wasn't able to pick up some insecticidal soap until yesterday.

Of course nearly the entirety of all three heads were destroyed by the time I got the soap, but I've already started some new lettuce two weeks ago and they're coming in nicely in a different windowsill. They're using the same MiracleGro soil as the sicks ones, so now I'm worried that the same thing will happen. I'm almost certain the spider mites came from the soil because I've had my windows close from March until now because of the unusually cold spring weather.

But the damage I can see in the pics is classic for both mites and/aphids so now that you can treat for them the problem should be resolved.

Just a guess as my romaine is already bolting down here, but I would think romaine could already be planted outside in your location since it is quite cold tolerant, much more than mites are.

Dave