2018 BRZ, MT, ~36k miles, JDL UEL catted header, OFT stage 2 tune, 93 octane gas, K&N drop-in filter
So a few weeks ago my car started exhibiting a huge loss of power, and also had an odd drone in the exhaust note. It felt like anything over ~20% throttle was ineffective. On the highway I'd lose speed when going uphill, and couldn't regain it until going back downhill. No CELs. I cleaned the MAF sensor and PCV valve, and also found that the clamp connecting the intake tube to the air box just after the MAF was loose and tightened it. I measured the resistance between the various wires on the two O2 sensors, and found 2.5 ohms between the two black wires on the upstream sensor, and 5.8 ohms between the two black wires on the downstream sensor, and no connection between any other combination. No changes in power yet.
I also took a look at a spark plug (photos attached). I chose plug #3 because several months ago I had a #3 misfire CEL pop up out of the blue while on the highway, but after clearing it everything seemed fine. The tip looks a little black, but it didn't wipe off on my finger like if it were soot. I went ahead and got some replacement plugs but haven't installed them yet.
Next I found an exhaust leak at the header, and ordered all new gaskets. When I installed them I took a look inside the header, and found that it was now catless. So I also removed the front pipe, and saw that it too was now catless. The remnants of both cats were crammed inside the front pipe where the diameter gets smaller, mostly blocking the exhaust. Removing all of that (and loading the stage 2+ tune) fixed the power issue, but now my big concern is finding and fixing the original issue before replacing at least the second cat. From what I've read, it sounds like an excessively rich fuel mix or unburnt fuel reaching the cats is what would cause them to fail, but I'm still not clear on why or if that's my case.
Here's a log from back when the exhaust was clogged:
https://datazap.me/u/kambroso/log-16...0?log=0&data=1
And here's a log from the other day:
https://datazap.me/u/kambroso/log-16...7?log=0&data=1
Any ideas of what could have caused my cats to fail, and whether I still have that issue? Let me know if I didn't log the proper items to diagnose this issue. If that spark plug does look bad to you guys then I'll go ahead and get those changed out, and let me know what else I should check. Thanks in advance!