I'd be cautious diagnosing trans problems on the Internet. The OP indicates "... has [started] acting up lately". Not, it always had this behavior. That suggests something changed where the symptom is new. High mile transmissions likely have deeper mechanical issues then fluid/filter change-outs will correct. I'm not going to hex the situation with the following comment but there is plausibility to the statement.
Changing out fluid may make things worse. It has been reported, on high-mile cars, when fresh fluid is introduced the properties are such that worn clutches, seals etc. do not adapt well where new and exciting problems show up. The problems may or may not be severe but even a proper servicing procedure may introduce undesirable results. No one can predict what will happen including a "VERY TRUSTED" mechanic. I am coming from direct experience with a Mercedes where hoping to gain a couple more years with a fluid/filter refresh proved a very bad idea. Bounce this concern off a BMW dealer or specialty shop and listen to what they advise. Or find a shop that will warranty the work.
Anything made can be made Better