Yep, Connors missed a lot of slams that would have helped his resume, but that's another story, not competition. However, it's not like he would have had a realistic shot in all of them. The AOs he missed during his peak are his best chances. At RG he would have struggled in the late '70s against Borg and Vilas, and in the '80s he could have lost to several players at the AO like Lendl, Mac, Wilander, etc. His biggest issue was he wasted a lot of chances in slams he did play, like making 3/3 finals in '75 and losing them all, in slams he had won the previous year. If you blame Lendl for that, the same should apply to Connors. From '75 to '78 he made 7 finals and won only 1 of those. Then he made 8 SF out of 9 slams in 79-81 and couldn't make the final in any of those. He had that renaissance in 82-83, if not his resume would have looked a lot weaker. Both Connors and Lendl have poor conversion when it comes to slam SF and F. They reached very easily the latter stages but struggled to go all the way.
I put Lendl above Connors and only below Borg and Sampras when it comes to Pre-big 3 open era. Agassi, I'd put in the same place as Lendl, despite lesser consistency/dominance, he won the four slams (+OG and YEC) and made the finals of all of them in a row in a very heterogenized era.