I've been having intermittent trouble starting my V6 2007 Outlander (215k mi) for the last 2 months. It cranks strong up to 600ish RPM and holds it there pretty well using the starter motor, but it can be sluggish for it to start and hold idle under combustion. Normally it just takes an extra 0.5sec of help from the starter before it lazily chugs to life but twice it has taken 10+ seconds while on the throttle before it started. One of those times it even died out coming off the rev and settling to its normal 750 RPM idle. Hasn't thrown a CEL, but my OBD monitor did catch it silently flagging P0171 - Bank 1 Lean. That makes sense imo because it feels similar to cold starting a 2-stroke without enough choke.
Seems to run fine at speed, maybe a touch down on power below 2000 RPM but I'd have to drive it back-to-back against a healthy one to know for sure. The weird thing is that it comes and goes, getting bad for a few days then running fine for a week. No difference whether the engine or air temp is hot or cold.
I've done an oil change, Sea Foam fuel treatment, Sea Foam upper treatment, cleaned the MAF, and run a can of BG44k with no appreciable difference.

My theories:
O2 sensor(s) - maybe, but my OBD shows O2 sensor, short term fuel trim, and long term fuel trim consistently nearly equal in both banks while running
Spark plug(s) - they're at least 100k miles old and should be replaced anyways, but not sure this is the problem because there aren't any cylinder misfires felt by me or reported by the car
Fuel pump - probably not because it's only ever Bank 1 reported lean and I assume there's only one fuel pump that feeds both
Fuel injectors - my first suspicion was a clogged injector, but doubting that because aggressive treatment with fuel sys solvents didn't do anything
Vacuum, something between the injectors and pump, ??? - a few other ideas that have crossed my mind but don't know enough of to rule in or out

Anyone have any thoughts or diagnostic tips to share? I love this car and don't see any reason it wouldn't run for another 30k+ miles if not for this, but can't justify spending tons chasing gremlins on a car worth $3k when it runs well.