General Rules of Thumb
The short answer to this is all Arrivals should be via Managed DES and with the PROG page displayed. As soon as you get high above your profile, use the speed brakes. If you have a tailwind, especially on the 321, be especially attentive to the descent. V/S means Very Seldom. Some guys love it and use it like a master, but your protections are gone in V/S. Use V/S out of 12,000, speed 300, down to 8000, and you will soon be flying 300 knots below 10,000 feet, thus busting the 250 below 10,000 rule.
"Open" XXXX (climb/descent) should primarily be used with ATC says "....and maintain" with the clearance. Intermediate descents, say 350 down to 310, due to turbulence at 350, is a DES not Open Descent.
I don't get too worked up over the math I watch the intercept path symbol and descent arrows, and level off point, to manage stuff. I don't have my head in the sand, blindfully ignorant either, but I am just not doing math equations for my descents. I might use "math" for a sanity check nothing more. I am not calculating decel points or things like that.
Also, it is easier to "get down first" then slow down once you level off, versus try to slow down, AND get down. The Airbus really does not slow down and get down well at the same time.
If in the airport environment aka "talking to Approach" and speed 210, go to Flaps 1. Downwind is typically speed 210. Base turn, speed 170-180, Flaps 2.
Watch the VLS with spoilers out, some instructors will tell you "it doesn't matter" if you airspeed enters VLS, others will break your kneecaps if VLS touches your present speed. Speed 210 with Flaps 1 largely mitigates that.
Again, general rules of thumb. Adjust for the operational situation at hand.