I'm kind of embarrassed to report that it was the heater! I thought of it at first but dismissed it to check last, if at all, because everything in the tank and sump looked fine, and when I glanced at it it wasn't obviously broken or puffed up or anything. It sits kind of dirty at the bottom of my sump. So I was checking all the fans, pumps, etc thinking maybe it was the DC pump that runs my closed loop. Whenever I'd plug it in I'd smell that fried electronics smell just a little bit stronger, but it ran completely fine! So Timfish happened to be in the area, walked into my house, right away pulled out the heater and found this:
So I think what threw me off is the skimmer aerating the smell from the water, so I kept thinking something in that section had to be what was wrong. The DC pump would've done the same thing to a lesser degree. I'm so thankful for ARC friends! I guess on the plus side, my skimmer pump got clean for the first time in 3.5 years.
I now have polyfilter running and all is well. What a stressful event!! I'm so thankful nothing died from the ordeal (yet?) except a little RTN at the base of some SPS this morning. I don't plan on putting another heater on the tank. I'll see how the temp holds up this winter.