Just thought I would update this thread.
TLDR version:
Dealer agreed to look it at again, diagnosed as bad booster, but didn't fix the problem. Dealer opened a tech line case without any resolution. Replaced the master cylinder myself, with little if any improvement. Found out engaging the parking brake makes brakes feel better for a minute.
Long Version:
While picking up my wife's Sportwagen from the dealer after they did a DSG fluid change, I got a chance to speak with the lead tech for a few minutes. Explained the situation with the brakes, and let him test out the brake pedal. Turns out they had another Rabbit waiting for an alternator so he fired it up and we compared the pedal feel. The other rabbit's brakes we're much better, almost exactly how I remembered my 2020's brakes feeling. He agreed there was a difference, and that they would schedule an appointment for a more thorough diagnosis. They would perform a bleed with their machine to make sure it wasn't air related and go from there. I did mention that I was about fed up and if they weren't going to do anything, I was going to replace the master cylinder myself. He said he had never seen a bad master cylinder in his 15 years, and that he thought it was probably booster related.
Brought it back over for that appointment, and sure enough, they did a bleed and found no air in the system. The tech's diagnosis was brake booster, so they ordered one. I told the service writer that didn't make sense, but that I would let them go through the process. The part finally came in and was replaced, but with no improvement in pedal feel. They admitted they didn't know what else to do, so they opened a tech line case and scheduled another time for me to bring it back. After another appointment, the dealer said that the tech line people could see nothing wrong with the data coming from the dealership. I asked how tech line was supposed to see something mechanical or hydraulic over a computer, but he didn't really have an answer. I went to pick up the car and tell the dealer to go to hell, but they told me that they were escalating my case up a level and that someone from tech line may have to come out and see the car. They said they would reach back out when they heard back from tech line. That was 3 weeks ago.
A week ago, I decided to just replace the master cylinder myself. There seemed to be a marginal improvement, but it's not perfect. I didn't have enough brake fluid to perform a thorough bleed, but it's at least at the same place it was before. I will give it another bleed this weekend to make sure there's no more air.
I did discover one interesting thing 2 days ago. Ended up parking in a different location for work and the space had a little downhill slope, so I engaged the parking brake. When I left after work, the brakes felt amazing, jut like my old car. However, after a few stops the effect wore off and the pedal travel increased again. I was able to re-produce that effect by engaging the parking brake each time I park or stop, but it always goes away after a while. Not sure what is happening here, unless the parking brake mechanism in the caliper wasn't self adjusting correctly and the piston was re-setting farther pack into the caliper bore than it should. I did reset the pistons back into their bore a few months ago in an effort to get all the air out. I'm not sure I understand this one.