I tend to agree with you that Rhavin probably is placed too high ... but RJ also made a point of showing that skill was more important than strength, look at Graendal, she's the second weakest Forsaken, but from the books was vastly more accomplished than those of much higher power!
And an another important point is that Graendal's fame came largely from her non OP skills. We knew she treated the minds the OP couldn't touch, and we don't know if the code of ethics of the AS let her use some forms of compulsion benevolently or if this was forbidden and she developed those skills, helped by her psychiatry background, only once a Forsaken.
Rhavin is largely described as a politician/diplomat. It's quite possible he never shone, despite his strength, as a channeler (the same can be said of Balthamel, for instance, and Moghedien). He surely was well trained, and surely he had to lend his Talents to tasks required by the Guild like everybody else, but it doesn't sound like he made a "OP"-focused career, the way Semirhage, Aginor, Lanfear or even Ishamael (as philosopher of the Wheel/Pattern/metaphysician) did. Demandred, like LTT, seems a bit of an unknown.. obviously both were very skilled channelers and it sounds likely they owed their ascension in the Guild to the merit of their service as AS, but as for the details we'll never know.
I do think Graendal should have had a higher strength rating given the comments from the books, or rather, that some of the other Forsaken should have been a bit lower as 3 doesn't seem too low for Graendal so much as Mesaana and Moghedien seem too high from the books. Semirhage was a little of a surprise, but not shocking. Mesaana as a 2 seems at least 2 levels too high and Moghedien seems like she should have been a 5, which also would have fit better with Nynaeve being her equal at one point, but still growing throughout the series without actually hitting her Level 3 potential. Similarly I would have thought the men would have been on at least 1 or even 2 more levels of the ++ ranks. From the books it seemed to me that the men should have been:
Ishamael, Aginor ++1
Demandred ++2
Sammael ++3
Balthamel ++3
Rhavin ++4
Be'lal ++4
Asmodean ++5
But to your point, Graendal clearly achieved her status through her career as a psychiatrist, despite her clear ability with the OP I find it strange that next to Moghedien she is the weakest of the Forsaken in strength.