It would probably be a big rewrite, so this is probably asking a bit much, but all the key items grant a 90 minute reduction in imprimatur regeneration time, right? And there are 6 coalitions with 7 total ranks to gain in each, for 42 coalition rank increases total. What if each rank gained reduced imprimatur regeneration time by 150 seconds, up to 36/42 completed (so, Magnate in all coalitions, or Legend in five and Probationer in the sixth, or a few combinations in between.) The same 5400 second reduction, but it tracks progress a little more intuitively and actually helps players who still have a reason to do their assignments.
Maybe imprimatur storage could be tied to the number of coalitions a character has reached Legend standing with, with the first three coalitions increasing the maximum by 1 each until the current maximum of 18 stored.
This would mean taking away the bonuses currently tied to the shovel and mattock cordon key items, but that's okay; why don't we use them for a different sort of progression that would make them significantly more valuable to players: have the bronze shovel and mattock cordons increase Ionis duration by 1 hour each, silver shovel and mattock cordons increase it by 2 hours each, and gold shovel cordon increases it by 3 hours, for a final total duration of 12 hours. Sort of a parallel to how medals work for Sigil, and a huge quality of life improvement people have asked for in the past, which would give players who don't need Ygnas or an Idris or Epeolatry a reason to care about coalition assignments.
Again, this is probably too much to hope for, it would require some rewrites to things I imagine, but why not dream a little.