"and me stuck waiting days or weeks (in Florida, in the summer) for replacement parts to come in."
The local Trane warehouse should have all parts -locally. It should never take weeks to get warranty parts. The HVAC contractor you are using is putting you on the back burner. Look up other Trane dealers from the Trane website and tell them your problem, other contractors wouldn't mind taking away an account from somebody. Next time when you purchase a new system, ask about investing in a 5-10 year parts and labor warranty. Since Trane would be the one paying for the warranty parts and labor, after a few times they would just send out a completely new unit, and get on the case of the dealer if the unit was undersized from the start.