Outsourcing the audio to a sound post facility is the best way to get a quality result and the sort of consistency you seek.
FCP doesn’t have sub frame edit accuracy or tools for dynamics & EQ that are a patch on pro audio systems, but most importantly, very few editors seem to understand audio well enough to get the consistent professional result that you may be seeking.
STP is a better tool, but as someone who does mixing on Fairlight and grading in FCP, I can tell you that exporting an OMF and getting on with the next edit will be a more efficient workflow than round tripping to STP to use a slightly better audio tool.