Why would Iran want to do it? Iran could absolutely control Hormuz* and get a nuke at will** under the deal. At that point it can avoid sanctions by threatening the Gulf. Or demand other concessions for allowing the Gulf to operate. That's what it tries to do now, but from a much weaker position compared to the deal timeline.

* Take the real life prewar Iranian missile and drone strength, assume Iran had an extra couple of hundreds of billions to pour into it, no import limits by the UN, a less restive populace, etc.

** Breakout timeline being a week at most at end of expiry timeline. Probably the only way to keep the Gulf independent at that scenario would have been nuclear deterrence.

>If not, why would they even bother to follow the agreement in the first place?

Because Iran wasn't anywhere strong enough at the time to risk a war, but the existence of the agreement and general developments would have predictably changed the power balance in its favor.

>The legality of what they do has never been a consideration

Well, if you decide something is legal, it's much tougher to change one's mind and stop it later.