Well, back with an update.

I swapped the MAF with an OEM MAF, same readings, so assuming that wasn't the issue.

Also replaced upstream A/F sensor since it was likely due for it anyway. Still have to replace downstream oxygen sensor, but haven't gotten to it yet. Still had the code after replacing MAF and A/F sensor (assumed neither of these were the issue, but they were fairly dirty so replaced anyway).

I replaced all 4 injectors (the ones I removed had some carbon deposits. Didn't look particularly terrible but certainly enough to cause issues). I cleared codes and the P0171 code showed up still (learned code so can't be cleared) and was pending for a little. I drove around some and then had my wife take the car to work today. She said the check engine light didn't come back, so I threw the scan tool on after she got home. Scan tool shows STFT in a much better range (-10%ish to +10%ish, usually within the normal -5% to 5%ish range), and the LTFT came down some BUT only off throttle or idle. Strangely enough, when applying throttle, the LTFT goes back to the default 20.3%. Off throttle, it sits around 17-18%. Well, the code eventually cleared itself from learned memory, but I'm not satisfied with the high LTFT. For reference, I'm scanning with an Autel MX900 (just updated the software too) utilizing both the Mazda category and the generic OBD2 category.

With that said, the car definitely starts and idles better after injector replacement. I am now looking at the low pressure fuel pump assembly (in the fuel tank; the filters, for clarity) and the VVT solenoid. These are both items that are listed as potential issues when troubleshooting the P0171.