I just think you are ignoring the evidence from the book that puts what you are calling "higher than average" a great deal more rare than you are giving credit.

I also think it's inaccurate to call Egwene/Elayne etc merely "Strong". Every shred of evidence in the book puts them at just below the highest echelons of female strength, with maybe a dozen or so women with more strength, out of thousands of women who can channel there are likely to be less than 100 who are stronger, and even fewer (if any new agers) with more skill. It's kind of like saying having a 175 IQ is just "smart" simply because there are a few people out there with a 200 IQ.

While I generally agree that Lanfear is practically unique, I don't think there is evidence in the books to support your claim that she is "several levels" above Graendal. It's obvious that she was stronger, but other than knowing she is weaker as Cyndane yet still stronger than Graendal we don't have anything to go on. We don't even have conclusive proof that Graendal is the next strongest of the FS, lots of hints, but it's possible Semirhage and Mesaana possess more raw strength (in fact I don't recall ever hear that Cyndane is in fact stronger than either of them), which is the only way I can really see Lanfear being more than 2 rungs higher than Graendal.

Personally I think you tend to make the Forsaken strength spread too wide (between each other). I see them as all very close (with the exception of Moghedien) in absolute strength, but with vastly different skill levels.

Semirhage probably being the top of the skilled heap, or perhaps Graendal. I say this because Graendal is unsure that Demandred is more capable than Semirhage in her tGS POV and she has no reason to be lying. In fact I would go so far as to say that Semirhage and Graendal were likely 2 of the most highly skilled women of their age, both achieved the 3rd name, both were world famous in their fields and both caused a great stir when they went over to the DO.

Lanfear on the other hand had to make a dramatic announcement that she was going over, and really did anyone care that a disgraced (if very strong) researcher who had no distinction other than being LTTs stalker ex-gf became Forsaken? Moghedien and Mesaana probably barely got notice when the went over... in fact I'd bet Mesaana means "Giver of Detention"

I don't think there is any evidence that any of the Forsaken feared or even really respected Lanfear all that much despite her strength, oh they were wary of her temper as none of them seem to relish open conflict and they all pretty much hated her guts (of course that's nothing unusual for them), but if she was really so much more powerful than any of the other women they would have been much less likely to openly stand against her, Semirhage clearly makes it obvious that she hates the woman and seems to have no fear at all.

IMO the skill levels (which the FS are clearly more impressed by):

Semirhage/Graendal
Lanfear
Mesaana
Moghedien

for the Men it's less obvious IMO but I'd say:

Ishy/Aginor
Demandred/Sammael
Be'lal/Rhavin
Balthamel/Asmodean

Sadly if Aginor had ever grown a pair he could have gone even further, but his general lack of anything resembling a spine held him back in this unscrupulous lot.