2017 Tahoe with very rough idle

[dksharp]

sounds like you just need a better diagnostic, like a misfire graph and/or compression test that would probably tell you a little more
the fact that the mechanic says to "trade it in" means either it's something they could probably fix easily and flip it or they don't even want to deal with it.
injectors on some of these trucks have a extended warranty period under special coverage which you could check at the dealer with your vin.
in any case you are on the hook for diagnostic cost. It could be beneficial to invest in a decent scanner, for this scenario and any future ones.

These are some good things to go after. A compression test could tell me alot. Especially if I have a stuck lifter. I also plan on shooting the motor with a temperature gun while idling to check for any temperature issues.