I would suggest taking your car to a local independent transmission shop. Hear what they say and likely have them do the work. Frankly they likely have more experience and expertise than the dealer. I suspect they are going to tell you it is going to be expensive, but less expensive than the dealer and likely will do a better job.
I am sorry to say this, but while I strongly suggest chaining the fluid and cleaning/replacing the fluid, it only works when you do it before there is a problem. It is a shame that most if not all manufacturers don’t seem to include transmission fluid changes are regular maintenance and I fear many transmissions are lost because they never get the maintenance they need.