@TrevorS, his sarcasm is necessary and very much to the point! This forum is not my life. You will have to accept the fact that I occasionally do pointless things like work, sleep or watch a movie.
Now to your question: Advisory VNAV has received a major rework months ago and was tested for quite some time during a lot a flights. By myself and by the people who volunteered for testing. Advisory VNAV (snowflake as well as blue ball) are only displayed while you're in a 3 degree corridor around the optimal descent path. And like with the glideslope of an ILS, the closer you get to the end of the descent path segment, the smaller this 3 degree corridor becomes. That's what you describe as twitchy. Back in 2017, the corridor was calculated waypoint by waypoint, now it's calculated constraint by constraint. It's much more precise and much less twitchy than before.
I can't comment on the T/D distance issue as I've never seen that problem. I usually just look at the green dot on the MFD and that moves totally fine without getting stuck.
I have this feedback:
when I set takeoff thrust, trim wheel fires up to its maximun setting (and alarms are activated); moving (simply touching) my Saitek trim wheel immediately, the problem is corrected
it happens only with the Airbus, so far
There is really no setup required other than making sure that both packs are running prior to takeoff, and all doors are closed. The crew needs to set the landing airport elevation at some point prior to descent, but other than that, the system is completely automatic.