1999 Bonneville 3.8L NA 163,000 miles

When the car is cold, there is little problem starting. After the engine warms to operating temp, it is very often difficult to start. It turns over fine, but cranks unusually long, fires a few times, then fires and runs OK.
CEL light is on, P0171, lean condition.
I have replaced the FPR, some bad vacuum lines, spark plugs and the upstream O2 sensor.
I have cleaned the MAF (but have not ruled this out as the problem yet) If I disconnect the MAF while idling there is no change idle quality.
Fuel economy is poor (about 16 MPG) combined driving.
Coolant level remains constant (full)
No drivability issues noticed.
I do know that the plug wires are not in the best of condition, and will replace them at next opportunity.

Can anyone help me pinpoint where the problem lies?

you check for vacuum leak with carb cleaner and listen to engine? replace the boot on the tb?

I would check fuel pressure both warm and cold. It'* possible that the pump is getting weak and putting out the pressure needed to start when warm, may be an issue.

I will check the fuel pressure as soon as I can get my hands on a gauge.