I had this problem, after looking at maf, it turned out to be the. 135 coil pack.
Is the last option to cause a p0173 code. If it's not the MAF, I'll try it
Look at the live data readings. Not just randomly change parts. The codes are only a small part of the jigsaw in most cases, not the end diagnostic (unless you are a dealer or garage, and thus thick as two short planks)
You can see values, but if you do not have a sample to compare at different rpm, load, etc... If you know where those values are, I'd appreciate it if you told me
Long term fuel trims are stored by the ECU over a period of time, so they don't depend on the current engine operating conditions. You can check them with the engine stopped or running and it makes no difference.
Nominally, the long term fuel trims should be zero. They start to depart from zero as the fuel delivery gets less accurate due to engine wear and changes in other items as they age. It's normal for them to wander a few percent from zero in a healthy car. What you're looking for is a value that has shifted more than, say, 10%.
You also need to compare the two banks. If the LTFT for both banks has drifted in the same direction, i.e. they are both at +15%, then the fault is something like the MAF sensor which is common to both banks. If only one has drifted off, or they have gone in different directions, then the diagonsis is very different (e.g. air leak that is affecting just one bank).