I doubt a flush product will make any difference. I have never seen any sort of varnish in any of the transmissions I have ripped apart over the years. The worst is the usual clutch debris in the pan. ATF is not subject to blowby, moisture, acids or soot. Unless you are trying to limp along a dying transmission, I would not recommend any additives. Mind you I am not a professional mechanic, nor do I play one on tv.
If you are getting some TC shudder I would suggest a quality group 3 or 4 based ATF, like Maxlife, Amsoil (I think they have a great product with a ridiculous price) or similar. I ran a group 4 in my Ram 1500 with the 545RFE. Its a product made in Saskatchewan, Co-op ATF SL, multivehicle 7cSt fluid. 9$ a liter and it did a great job in that trans under some heavy towing. Its also doing just fine in my AT2 transfer case and various PS systems.
I am not sure how much fluid is left in the system for sure or how much the TC retains, if any. When I did my TB in 2014, I used about 13 or so liters and the fluid was bright pink pumping out. Dipstick checks to this day still show it bright pink and smelling as good as ATF should be. The 4L60e in my 1993 K1500 used maybe 11-12 liters if I recall to flush out the nasty dark brown ATF. Dipstick check shows it nice and pink still.
Don't over think this, just pump out the old and pour in some new stuff.