A RDM Equivalent to BLM Triplecast

[Archwizard]

Well, yes in that specific case of course.
But I think it's a broader issue of casters and movement. Each DPS caster has their own strengths and weaknesses on this topic, and their own tools. DPS uptime comes from integrating this element in the rotation too. In a way, I fear that it would be walking the path of over simplifying things. The addition of a generic "press here if you need to move" button to each caster would be a bit underwhelming, wouldn't it?

I 100% agree -- at least as far as each caster dealing with movement periods in their own ways, giving them their own identities. Otherwise you may as well remove Ruin II and just make Triplecast a role action.
Of course this does mean that BLM is kind of the odd man out in this regard, I think, since its movement has been woefully crippled without popping cooldowns that may also have been used for raw DPS, but hopefully some of the changes in Shadowbringers have addressed that to an adequate degree.

That said, while I do agree with such things as adding to a job's skill ceiling, it's a very delicate balance, particularly when it involves creating new tools where the entire fact of their availability to players can determine the player's skill.
For instance, to use your example of Engagement, while it is a "safer" option than Displacement, it could be considered something of a DPS trap tool by some players (particularly ones who think they're better than they really are or want to copy top-parsers, and will persist in using Displacement instead, continuing the trend out of spite), and "optimal" players will find ways to use it as little as possible (as you yourself have demonstrated); same case with Scathe for BLM. In both cases there have been (already!) cries for outright replacement.
On the flip-side, tools like Apocatastasis were primarily used by high-end players at very specific points in a combat engagement, and relatively ignored at lower points in play (which I expect is technically also true of Addle, save for the new players who think role actions are supposed to be part of their rotation and spam it on-cooldown alongside Drain and Break).
When you have tools that aren't used across all skill levels, it can feel something of a wasted space to one group or another. That's what makes something like Triplecast particularly appealing, since "optimal" players will find creative uses to get the most out of it and everyone else can still get their money's worth without feeling a loss.