Transmission warning on my CTS, dealer wants a fortune, any alternatives?

[keatonketteridge]

I drive a 2016 CTS and last week the transmission warning came up on the dash along with a message about reduced performance. The car still drives but it feels like it's holding gears longer than it should. I took it to the dealer and they quoted me something that made my eyes water, plus they wanted to keep it for over a week just to confirm the diagnosis. That's not realistic for me since it's my only car. I found https://gearboxrepairsharjah.ae/brands/cadillac/ while searching for options and they seem to know these gearboxes, but I've never used an independent for transmission work before. Has anyone here had transmission work done outside the dealer on a Cadillac? Did it go smoothly or did you run into issues with parts or software? Any advice on what to ask before handing over the car would help.