Ask the dealer if he also sells shares in the Brooklyn Bridge. If you read your Owner's manual, you will fing that the transmission on a Toyota has a very long service interval. And that same manual calls for DRAINING the fluid, and changing the filter, not flushing it.
The belt bit sounds like hogwash; I have never seen a mechanic pulling a belt off just to look at it, unless he was doing long interval, warranty-mandated inspection. Belts are normally inspected with every major service interval (10,000 miles); my Toyota dealer does this, without removing it. It's included in the service package price.
A typical Toyaota transmission calls for a fluid drain and filter change at 6 years or 60,000 miles whichever comes first. I would do it more often, such as 40,000 miles, but DRAINING the fluid.
Toyota calls for belt inspection every 10,000 miles or 12 months. If your belt is cracked they should be able to show it to you.
Please read your owner's manual; it supersedes any nonsense a service writer tells you!!