Is $650 to replace rusted furnace vent connector pipe a fair price?

I am a homeowner trying to determine the best course of action to correct what appears to be a condensation issue in my furnace vent connector pipe. I recently had the b vent cleaned and inspected by a chimney sweeping company. The b vent is all in great shape, but they found the connector pipe to be completely rusted inside and there is what they described as efflorescence in the connector. There has also been what I now know is broken up rust and the same efflorescence looking material on top of the furnace cabinet. My furnace is a 2 stage, 80% Ruud, installed in 2019, with a minimum BTU of 52,500 and a maximum BTU of 75,000. It is currently the only appliance using this vent system. My house is a 2 story home with a conditioned basement and attic. From the furnace, there is currently a single wall vent pipe that connects to the inducer fan outlet. The fan outlet is 3” diameter that has an increaser to 4”. This 4 inch connector pipe is about 20 inches before an elbow, which then goes to a Y pipe that increases from 4” to 5” before tying in to the main vent. The main vent is 5” b vent, starting from the connector pipe has a 6 foot horizontal run to an elbow. From this elbow it runs from the basement through the roof. This vertical run is approximately 27-30 feet. I do not believe the chimney company's proposal will actually solve anything after reading up on venting sizing tables. The HVAC company does not believe there is an issue and recommended swapping out the vent connector for new. I am wondering what the best solution is. Technically, it looks as if my entire b vent should be replaced with 4 inch b vent, but I am wondering if a safe and reasonable alternative would be to replace the current single wall connector pipe with 4 inch b vent pipe. Neither company thinks the double wall vent is necessary, but it seems like that is the appropriate material for this job. I am also wondering if changing the current horizontal 5 inch b vent to 4 inch b vent would help reduce the risk of condensation even further.