"Interestingly, by his thinking, the Constellation being the 10th design would suggest the basic saucer + secondary hull + nacelles had been around for a long time
Not necessarily, IMHO. Assuming the yellow production sketch is from the early beginnings (which I think it is), Matt Jefferies suggested the A-ppendix to be applied to ships that had undergone modernization or modification, not (yet) as a commemorative appendix to honor the achievements of a previous ship (like NCC-1701-A in ST IV).
Therefore the USS
Constellation's registry number could simply honor the achievements of a previous cruiser (unknown 10th design) without the necessity to wear an A-ppendix, as this kind of nomenclature hadn't be established yet by the time of TOS.