I agree with your basic premise about depth of women's clay play in the mid to late seventies but you overstate it a bit. We will agree that European players were comfortable on dirt, and American and British players were not. But I think you miscalculate Down Under. Both men and women were often very comfortable sliding on clay because they had a lot of clay courts in Australia and New Zealand and both boys and girls had plenty of access in large parts of the continent both rural and including Melbourne . Its no surprise that Court, Goolagong, Turnbull, Laver, Rosewall, Stolle, were all great movers on clay even early in their careers.
I will also agree that depth was decidedly better on clay for women in the mid eighties to early 90's with the swing toward European champions. The seeds were planted however with Navratilova, Jaeger, and Mandlikova in 1980 .
I don't think that I am overstating it to be honest. In all my years of following tennis, there has never been a period when the strength depth on clay in women's tennis has come remotely close to that in men's tennis. Clay has always been the surface with the biggest gulf in depth between the two tours. It's true that a lot of those Aussie players grew up on clay-equivalent surfaces such as antbed where they could slide. But firstly there many more of those Aussies that were comfortable on clay to contend with in men's tennis than women's tennis, and secondly on top of that in men's tennis there were also far more European and Latin American players brought up on clay to worry about as well.
Evert's first 4 RG titles came when the draw size was 64 players. I agree that the women's clay court field was better in the 80s than the 70s. But it was still wafer thin compared to the men's field on clay at the time. In the men's game the big guns had many, many, many more dangerous clay court specialists or players brought up on clay to worry about in the earlier rounds, whether it was Juan Aguilera, or Cassio Motta, or Martin Jaite, or Hans Gildermeister or Kent Carlsson and the list goes on and on. A similar list in any women's field in history would be very short in comparison.
Admittedly in modern day men's tennis the number of clay court specialists have declined as things have become far more hard-court centric. But en-route to Nadal's most dominant RG campaign in 2008, he faced Bellucci in the 1st round and Devilder in the 2nd round. Now those names don't sound impressive and he dispatched them comfortably (thought Bellucci was superb for the first 2 sets), but they were opponents who were both raised on clay and regard it as their favourite surface. Similarly he has faced opponents like Zeballos, Bolelli and Gabashvili in the early rounds at RG, again brought up on clay and counting it as their favourite surface.
And then even before the semi-final stages, where are the equivalents to the likes of Almagro, Fognini, Monaco in Evert's era on clay in or any women's clay court era in history? There just hasn't ever been anything to compare to that.
Now because of the lack in depth in women's tennis in general, the first week of majors on any surface were usually a leisurely stroll in the park for Evert, Navratilova, Graf. That continued into the 00s. But on no surface was that more true than on clay.
To be honest I might argue that Borg's clay court achievements are more impressive than Evert's, again that he had to contend with significantly more strength in depth on the surface across on the board from right from Round 1 to finals day.
I don't rate Jon Wertheim that highly as a tennis writer, but he once summed up this perfectly. There has never been the same clay court culture in women's tennis as there has been in men's, and because of the significantly reduced depth and number of dangerous specialists in women's tennis, surfaces differences have been a less important factor. That explains why the 5 greatest female tennis players from the past 50-60 years have won every major on multiple occasions.
Therefore amongst that backdrop, a male tennis player ruthlessly dominating on clay is always going to be a better and more impressive achievement than a female tennis player doing likewise.