I've never had a reliability problem with my used G19 of 5 years, so the cycling internals have been left alone.
I don't think the trigger offers enough resistance to "fumbling", so I installed a New York trigger. Feels about the same as stock, but better resists a misplaced finger while grabbing in the dark (my house gun). Also helps prevent broken trigger springs, which I've seen a couple times.
I'd put Trijicons on if I could convince myself to spend an extra $100, but I hate the fact that they were out over time. Still, a good idea.