Well, here is a thought... not to take anything away from majors, but you do not actually need them to separate (or to group, as I prefer) the top players. I explain: You could use all the other measures (weeks as #1, total titles, winning %) and etc and basically arrive at the same conclusions that you do when you count majors.Again, I am not saying that majors are not important, I am saying that most metrics of "greatness" are redundant. So better than to look for majors substitutes while comparing players from very different eras, you could simply do the opposite and ignore majors all together (just for the sake of this comparison).
Yeah, I hear you. I like to look via different lenses or metrics, while favoring none in particular. I do think that majors and weeks at #1 are the two best overall indicators of greatness, but both have problems.
Different websites have different ways of trying to encapsulate everything. For instance, Ultimate Tennis Statistics has their GOAT score (Open Era) which you can adjust with different emphases. I don't like the raw GOAT score because it overly privileges longevity and quantity (of titles, etc), so players like Connors and Lendl rank higher than Sampras.
In the end, ranking players is a judgement call. There is no perfect, objective way to do it - at least no way that both takes into account all factors (which the GOAT score does a decent job doing), but weighing them properly. In the end, the subjective factor is in weighing those factors. I see no problem with it - it is what makes the conversation interesting.
On a side note, Tennis Base had weeks at #1 for pre-Open Era players, based upon their own ranking system. I can't remember the exactly number, but Bill Tilden had something like 800 weeks as #1, hundreds more than the next guy on the list - who was Laver, who also had hundreds more than Roger. Of course being #1 in the 20s and 30s was quite a bit easier than it is today, but still....just another metric to look at and add to the mix.