The two things that the Reliants and Constellations have in common to me are their large shuttlebay doors and volume.
The one further thing they share is twin "impulse deflection crystals" (by virtue of the Constellation
having two impulse engine sets, and the Miranda
having those blue domes both atop and below). Perhaps impulse acceleration is another thing that was lacking in the Constitution
-refits and helped seal their fate?
The Constellation class were never a back-up plan for the Excelsior. The USS Constellation itself went on a one year trail run even before the Excelsior had left the shipyard.
This would be no contradiction of the backup idea. Naturally, the "safe" alternative would reach trials stage faster than the "risky" one!
Interestingly, the Constellation
herself is listed as still undergoing warp trials as of ST6:TUC, in a background graphic that admittedly cannot be read on screen but OTOH is meant to be a "serious" in-universe readout for a rare once, rather than a combination of gibberish and in-jokes. Might be that the "safe" alternative wasn't so safe after all, and the four-nacelle configuration presented so many problems that the Constellation
was in fact an expensive failure in the end.