Me, I wouldn’t flush. Tried that once and lost the Tranny 20k miles later. Drop the bottom and let the crap out no problem. A flush pushes all that crap around even more.
On my truck I never had touched the tranny until 170,000 miles. It started slipping once every few months or so. Not having been serviced before (so more gunk), it seems the consensus is to drain and fill a few times, with a few hundred miles in between.
That was my plan. I drained it, put in a new filter and cleaned everything. And tightened down all the little nuts that are holding the electronics in place. Then filled it. A few thousand miles later I haven’t done that again as I had planned. No slips yet though.
My truck had a bracket that was VERY difficult to drop the pan cover past.