Laver is a legend, but 3/4 majors were on grass when he won both his CYGS. Everyone you're listing had to contest the CYGS on three different surfaces, except Borg and Connors early in their careers.
Indeed three of the four Majors were played on Grass courts. However, the grass courts of Wimbledon, Westside Tennis Club and the various venues used in Australia (Milton, White City, Kooyong) during that era where very different and played very differently.
Now that was mitigated to some extent by the fact that the dominant style of play in those days was Serve and Volley - but it was so because of the Grass courts.
The modern Grass plays a lot more like hard court ... more consistent bounce is the main thing ... so different playing styles can succeed on Grass.
Imho, purely from a playing conditions pov, it was much harder to achieve the GRAND SLAM in Laver's time than it is today. I don't think the playing conditions - in terms of playing surfaces - is anywhere near as important these days.
But there are other factors which make achieving the GRAND SLAM very tough now - in particular the level of the competition and the significantly greater scrutiny that the top players are placed under.
That's why I have always said that any player who can achieve the GRAND SLAM now would factor in any GOAT discussion.