I think you have a situation where a load analysis would really pay off. Both "bigger is better" and "big AC is worse" is too general. It is obvious from your experience that the A/C is not sized right for your house. Even bumping one up a half ton for a total of 5 tons in Georgia for 3500 sq ft may not be enough when you like to keep it at 68. You also need to get your ductwork looked at, especially if you increase the size of the A/C units. The main benefit of the variable speed furnace is that it can extract more humidity as the air flows more slowly over the coils and will also increase the SEER of the units. I'm surprised that quote 3 can give you 16 seer with a single speed. The total increase is for two variable speed furnaces, so the price increase may not be that bad. As you say, the new standards for the South have phased out 13 SEER this year, so Quote 1 is not valid.

You are right, your condenser does use R22. It must be the last of the R22 condensers Carrier manufactured. You should not sink $2500 in repairing the condenser. My suggestion is to buy a Carrier condenser and coil. No advantage to introduce another manufacturer's equipment unless you can't find a competent Carrier dealer. You should change the line set also. You can't add a 2-speed condenser to a single speed furnace. If the furnace is working well then there is no need to replace it. The furnace is probably over sized for your climate.

Coastal or inland location?...I assume you are being quoted AC with air handler and perhaps a heat strip if needed for your climate. Depending on models quoted, Carrier and Bryant are identical. You want to be certain you are getting aluminum coils. I would also suggest getting a quote on a Trane or AmStd XR16 with var speed air handler. IMO

Attic space is tight 'cause it is a Florida house with "low" roofing. The quotes are EXTREME. If I had to fashion a reason it would be due to this (tight space). Duct work is primarily a labor intensive job working in of all places a hot humid attic made worse by the building it occupies. (worker injury, house damage, yeah there's a charge for that...) How extreme? Like rip off the roof to get to the ductwork extreme? It's an example why I never buy a house with a small tight attic. A lesson as it were. Can't even tell you how many of those kinds of jobs I walked away from. More like run, but well you know... if you don't ---about to find out. Other jobs I like to run from: homes with two HVAC units in a tight attic in which one of the units has to be removed to get the other one out. The one that's broken is always the one in the back. The stories I could tell, Oh dear Lord... I service the Katy, Texas area