Small adjustments is being optimistic at this point.
Anyway, this is probably not fine for a few reasons.
DRK's damage is fine. It's more than fine actually, all the tank damage is so close to each other it comes down to a per encounter and personal comfort level on which tank to take. DRK in particular, with proper raid buff utilization with Edge of Shadow, can put out very respectable ADPS numbers, and adjustments to any of the core skills or even the MP regeneration rate can have some pretty wide reaching consequences. Even boosting Salted Earth by a flat 10 potency is in actuality a 60 total potency buff per enemy. Regardless, you're never going to hit enrage because you brought a DRK. Offset that very minor DPS discrepancy in the tank role by optimizing Edge, and TBNing for your healers to throw out more of their 1 button nuke skills, the raid DPS will increase either way.
On the other side of the coin though, if you did make Abyssal Drain into a GCD action, you've actually nerfed DRK. Stalwart Soul is incredibly powerful, not because of it's potency, but because it generates 600 MP and 20 Blood in 2 GCDs with a potency gain at two targets vs Souleater combo's 3 GCDs. This is why Stalwart Soul's actual potency is low in comparison to other tank AoE combos. While other tanks only have one AoE dump option that will consume resources, DRK has two via Flood and Bloodspiller at 2, Quietus at 3+, meaning the actual burst damage is quite high. This is without talking about Quietus under Delirium giving more mana than usual, Living Shadow AoE scaling, etc etc. Even outside of offensive based skills, combining this with the passive MP regen, you can TBN, do three Unleash Combos, and have generated enough MP to TBN again on cooldown, which I think is more advantageous than healing outright due to TBN's power right now. We don't have any real multi-target fights in the tier, but this AoE kit advantage was huge in content that does have multiple targets.
When you add a 3rd GCD, all you've really done is add unneeded self-healing, for the most part, and delayed the MP and Blood acquisition. While I do miss Abyssal Drain being a dedicated GCD, making it into a combo action without considering the benefits Stalwart Soul's quick 1-2 regeneration gives us won't make DRK better offensively or defensively. Maybe instead we can lean into oGCDs like Abyssal Drain and Carve and Spit having multiple charges like on MCH.