If you didn't touch the timing previously, then don't bother looking there.
You've double-checked the plugs, so they're probably good. Nothing stands out in your narrative.
No codes means the computer isn't picking up a problem so look for issues that are not on sensors.
Plug wires:
1. one of mine was really worn. Somebody had previously left it twisted so it had worn against the frame mount. The rear spark was going to both cylinders. Replace and reroute did wonders.
2. Very difficult to be sure of solid wire to plug contact on the rear cylinder. It took several tries before I was sure. Actually bought the tool for next time, just because.