RDM is pretty much volleying between two spell rotations to build up the gauge in order to use melee combo. In terms of overall "stationary" mechanic RDM has the least to worry about when compared to BLM and SMN.

To a RDM they can Jolt and then proceed to move as they cast another spell, they can even use Swiftcast to bypass the first Jolt and use the first combo spell in hopes of proccing a ver spell.

They got a decent instant offensive ability as well, Fleche 420 potency every 25 second. Along with Corps-a-corps and Displacement 130 potency each, 40/35 second recast that give them added mobility.

Got a reliable "Acceleration" spell to force a ver proc and as mentioned for RDM the only momentum they lose is being unable to use a ver proc.

BLM and SMN have a lot more to lose if they lose momentum. If a SMN blast off their Trance, it ticks away and they could lose it after the 15 seconds is up, it becomes a race against the clock to use it once active meaning it's harder for them to ge ta full use out of it compared to RDM.

Perfect example, let's say RDM has 100/100, SMN has Trance ready. SMN uses Trance against the boss and 5 seconds in the Boss becomes untargetable. At the same time the RDM jumped in to use their combo but were only able to get 2 steps in before the boss disappeared.

What you are left with is a SMN that lost 10 seconds of their ability they built up compared to a RDM that still has 45/45 leftover meaning they only need to rebuild the gauge from where they left off and fire off the combo again.

BLM does have a similar issue because they have abilities on timers as well so if they time it wrong they face losing a lot of DPS either by holding it to long or using it at the wrong time, this is the one thing RDM doesn't suffer from which give them a lot of flexibility in all combo because they can be gunho about how they act, they have no need to hold anything during a boss fight because they lose nothing if they time a combo wrong, maybe they lose a ver, maybe they don't get the full melee combo off, but they always know they have a guaranteed dual cast along with only losing what they spent on the combo.

The other thing with SMN, they have to build up to that Bahamut Primal, so it's one trance, two trance and then Bahamut Primal, but this also means that in order to get to that point of dealing the big damage, they could lose out a lot if again they time it wrong. With that in mind maybe SMN's need an ability that allows them to recall the aether so that whatever they couldn't spend is put back into a pot to Summon Bahamut Primal again. Granted that could be a tad tough so my other thought is maybe do away with the time based mechanic of Bahamut and instead have spells cost "Bahamut Aether" and once the aether is spent the Primal leaves.

So The Primal would remain as long as you have aether, and there would be an ability like Flare that uses up the last bit of aether you may have, but at least this way If say you Summoned Bahamut and now the boss can't be targetted, you don't lose out on a huge amount of DPS. Instead you still have said aether to spend, where the RNG can just be focused on Ruin IV proccing rather than the short duration on top of it.

Maybe that's the issue, the Trance and Bahamut egi are to focused on two mechanics that force this period of burst damage if not timed right.

With NIN we saw that it was possible to disable the use of certain abilities while under a certain effect so the Prime could do the same thing. only grant access to the Ruin Spell, Tri-Disaster and Enkindle.