I think the problem is that DRK no longer has a defensive identity.

WAR is the tank with high-uptime, sustained mitigation (Veng/RI/Rampart/Thrill add up to ~75% uptime; that goes above 100% uptime if using IB).

PLD is the 'selfless' tank that mitigates for other people (PoA, DV, Cover, Intervention particularly with Ramp/Sent).

DRK still seems to be stuck as 'the magic tank' in a world where that isn't a meaningful distinction.

I would like to see DRK instead become 'the tankbusters tank', with low-uptime, high-power, high-frequency cooldowns. Dark Mind still feels really good in fights where it works, and I would build on that:

Bump DM to 20%/40% with DA.
Make Shadow Wall a physical-only clone of DM; 20/40% depending on DA, with the same 60s cooldown.

I think that would give DRK a strong defensive niche, and isn't really out of line compared to WAR uptime and PLD party mitigation.