Hey Lemon, this is the official procedure that a US dealer would follow:
https://static.nhtsa.gov/odi/tsbs/20...63890-9999.pdf. The transmission fluid you want is called Mobil 1 Synthetic LV ATF HP, part number 19417577 in GM's system.
It's a single flush process with a purpose built flush machine (as Cyrus says, the triple flush was an older attempt at removing the previous, even worse fluid, but the latest 2019 fluid supersedes them all, actually works and no longer requires the triple flush).
Not sure what equipment other shops will have, but even if they don't have an equivalent flush machine, you should go and get it done
right now, expunging "only" 99% or even 95% of the old fluid will still be highly relevant. If the shudder returns after the flush, unfortunately you'll need a new torque converter, GM part number 24290214.
Getting a transmission tune is also advised, although not required, to extract maximum performance out of your 8L90 and to make 101% sure the shudder never returns. Plenty of remote tuning options even here in the forums, my tune is from Ted Jannetty at jannettyracing.com, and it's spot on (I have the same 2018 with the same trans as yours).