I was in a similar situation. Just purchased a High Mileage 2012 Tahoe Z71 and just started to begin the maintenance of the fluids. I had the car fax report so I had a good idea of the vehicles history before buying it and all the fluids have been changed multiple times. It really shows in this truck. It feels more like it has 20K miles rather than 143K. I just read about your needing the transmission, although that isn't what you would hope for, at least you will be getting one big potential problem with high mileage truck out of the way.
Ya, it’s sucks, but not the end of the world. I budgeted for some substantial repairs, but didn’t expect to need to burn this much on just one.
But ya, it will be good to not have to worry about it for a while.