I vote with drop the pan. I had a problem with 65E at 80K miles roughly. I changed fluid 3 times in 12 thousand miles or so by dropping the pan and changing the filter (had done it at 65K miles). I did not change filter on repeat flushes. Keep the rubber covered gasket from the original unless someone else ha switched already. They help cool the transmission. They are meant to be reuseable.
I replaced about 7 quarts out of 12 by dropping the pan. Next time I replaced 7 quarters out of 12. Third time I replaced 7 quarts out of 12. 41%*41%*41%=7% of original fluid left in trans. Probably same as a flush would have done in one run.
Replacing part of the fluid is not as likely to make a quick change to some clog or crud in the tranny. Buick Service Manager did not recommend a flush. He verified with another service manager who do flushes.
I'm at 135 K and going fine.
Dual auto air on both