Shadowbringer

I can buy the standard version and then later update the collector

That is correct

There are separate collectors editions for ARR and each expansion. All three (and presumably all four soon) are available as separate digital upgrades if you started by getting the standard edition. That's how I got the ARR Collector's Edition, by first buying the standard edition of the game, and then buying the ARR CE upgrade. There are similar CE upgrades available for Heavensward and Stormblood, and I assume there will be for Shadowbringers as well. Each one has its own rewards and is sold separately, though, so it's three (and soon four) different purchases to get all of them, or if you're just getting one, be sure it's the right one that gives the reward items you want. The Shadowbringer CE upgrade won't give you the Realm Reborn, Heavensward or Stormblood CE items, just its own.

To that end, Heavensward is probably the most worth it out of them all. With the Removal of Fantasia from Veteran Rewards, you get one for finishing the Final Quest of base game ARR. So having one extra one on every character without having to hit lvl 50 and finish MSQ is fantastic, especially if you get invested in your job and don't want to reroll to be a new race, ect.