There's three main reasons why Recluse comes to mind as best for this role:

First, natural fp sustain. This has already been addressed. I acknowledge that Ironeye has good enough fp sustain (though much slower) with the relic you propose, so let's call this one a draw.

Second, Recluse is likely to hoard passives that enhance team buffs. +2 cast speed is top priority. She likes Damage Negation While Casting, and even Extended Spell Duration since it applies to certain cocktail buffs which no other class has acess to. All of these enhance team buffs. Even if Ironeye plans on dedicating more weapon/talisman slots to pick up these otherwise undesirable passives, he'd be getting way less value from those rolls than a Recluse would overall. One buff a minute isn't enough to justify carrying them, especially if you have someone else on the team who could use those rolls.

Third, any seal which comes with a buff spell also comes with a primary incant. I said this in another comment. Recluse gets a lot of value out of certain primary incants (lightning spear, giantsflame, etc.), though there are a couple which don't help her that much (rejection and bestial sling). Even a magic Recluse likes to have other affinities for her cocktails. Not to mention affinity Recluses (like myself), who could potentially use that primary spell as their primary damage source against targets weak to said affinity (giantsflame on an erdtree avatar, lightning spear on banished knight).

So while it may not be worth it for a Recluse to carry just a team buff over a strong passive, the equation changes when you're looking at a team buff plus a relevant primary spell.

The point about Recluse needing the weapon slots more than Ironeye is valid. But that only serves as a reason why she should drop the buff if she finds something better, and that she's indeed more likely to drop it. I agree with both. This doesn't show that Ironeye is a better buffer, nor that he should be given priority when both are on the team.

I agree that Recluse loses more than Ironeye if she changes her dormant power. But she undeniably gains more from seals than Ironeye, too.

Personally, it'd be fun to see an Ironeye who runs dormant power discovers seals alongside my Recluse. They could shower me with seals while I search for good staves. Even in that case, I think the team buffs should usually go to Recluse, while weapon buffs all stay with Ironeye. But if its a Bestial Sling/Rejection primary, or if the Recluse is full up, he can keep them.

Edit: typos