Questions on my wanky brakes and fixing them honestly

I love my 2004 Buick Rainier, truck/SUV..A SVU on a truck chassis..

Good on mpg for an 8 cycl...and powerful engine.2022 bought it but it's always had a weirdo thing with brakes.Stops well, strong, but when approaching a light, stop sign , very slowly, under 5 mph, the brake pedal drops, then pops up. So far I've read 3 separate reasons..going to repair shop and don't want a huge bill for a quick fix, nor get ripped off when they see a woman..Any suggestions please??