Hmm. Mine might be a close, though not perfect, match for you.
Not a harem per se, but there is a MFF thruple for the central relationship, and there are lots of strong female characters throughout the series, many of whom are stronger than the male character.
Now, the two women in the thruple are not stronger than the guy overall, but that's based on situation and his background, rather than a difference in potential. One of them starts off strong and experienced, the other one is starting as basically a civilian shoved into a not-civilian-friendly scenario, but she gets training and rapidly grows in strength when pushed.
Of course, a lot of it is slice of life/semi-cozy, so direct power isn't always the most important thing, and Kazue has plenty of indirect power. Partly because she has no idea that she has that much indirect power.
Anyway, back to the point: I am reaching the end of volume 7, and the MCs still can't compete power wise with a nine-tail, and the two nine-tailed kitsune who are frequently in their company are far enough beyond the baseline for having nine tails that they are going to remain stronger for a long time, and the stronger of them should remain stronger even past the end of the series.
If the FMCs having a lot of real agency will substitute for having more direct power than the MMC, then this might work for you.
Seriously, the world is filled with strong capable women. I just can't help but add them in almost reflexively; I have to remind myself to not make too many of the characters women.
