Frankly, it's probably impossible to make PCT not be the strongest job in Ultimates just because it checks every possible box for benefitting from how they tend to be designed.
1. It's damage profile is burst-oriented, with the bulk of its damage being in cooldowns that will keep ticking, while the filler GCDs that it loses are comparatively weak
2. It has GCDs that can be spent during downtime to create a little bit of extra uptime for itself
3. It has a lot of incidental cleave on its most powerful abilities, giving extra benefit against 2 targets
4. Swift -> Rainbow Drip is a strong option for right before the boss goes untargetable because its long GCD is completely negated
The problem is that Pictomancer isn't some middle-of-the-pack job that then becomes strong, suddenly matching or slightly exceeding the heaviest hitters, that would make DPS checks slightly more lenient if you brought them along.
Instead, they go from being the strongest job to the even-more-stronger-est job, completely eclipsing everyone else and making the job balance go from bad to embarassing.
How would I nerf it? I would walk back all of the potency changes that they gave it after the media tour that they were too proud to admit were a massive overshoot.
Creature Muse 1000 -> 1100
Mog 1100 -> 1300
Madeen 1300 -> 1400
Rainbow Drip 900 -> 1000
Star Prism 1300 -> 1400
Hammer 520/580/640 -> 560/620/680
RGB(W) 400/440/480 -> 440/480/520
CMY(K) 700/740/780 -> 800/840/880
If they'd just admitted they messed up and reverted all of these to their previous, more sane numbers, they wouldn't have needed to add all of the other potency bloat that we've gotten since 7.0. Would Picto still be the strongest job in Ultimate with those numbers? Almost certainly. But it would be close enough to the realm of mere mortals that taking a BLM instead of a PCT is like taking VPR instead of NIN, or SAM instead of MNK: a slightly suboptimal choice that a player can make for comfort or enjoyment, not a difference so stark that it's essentially griefing.