I would stick with this for now, that is why you went to the garage, the rest - their findings - may or may not be true but many garages are 'willing' to make money - same goes for the Volvo dealer when they see an old car
Did you recently had a long ride perhaps driving through mountain areas with lots of uphill? If so, the old transmission fluid may have overheated and it's no more efficient. Replacing even part of the fluid can't hurt, you'll need a garage to undo the return hose, most garages can do that. The clip may break so it's good to purchase first a new one with seals from Volvo ($30?). With the fluid this job should not be more than $100, it's simpler than an engine oil change. As mentioned no power flush (many garages do this)
Keep in mind, the transmission may have other issues. I remember some models had a problem with a seal failure allowing coolant to leak inside the transmission, a coolant check for oil contamination would be a good idea just to be safe (just check inside the coolant reservoir)
The thermostat doesn't give a temperature, rather the coolant sensor does so. Here's how to check the thermostat Frankly, if the engine warms up quickly and the gas mileage is fine, leave it like this for now.