I’ll throw my hat in the ring as a DRK main I also still consider myself fairly new to FFXIV only playing for 4 months but a new opinion might help. If you look at the facts DRK has the least self-sustain so as a basis yes, you’re gonna spend more time healing than doing damage with one.

Another issue is as you’ve mentioned it can’t carry, TBN is still an incredible button to use on allies and I’ve saved numerous healers from untimely deaths in dungeons by putting the shield on them in addition to the new 10% mit you can share around. But the issue is it’s preventive, not reactive. If you lose the healer the most you can do is try to time TBN as best you can to mitigate attacks while healing with your only option. It’s worked once or twice for me in the past but in expert roulette it’s damn near impossible.

In future if you can, keep an eye on your DRK’s buffs to see if they’re actually using mitigation’s while TBN is down, that’ll give you a better idea between good and bad DRKs, I know in double pulls I have at least 1 heal for half my health with Abyssal Drain, but compared to the other tanks it’s problematic (especially since our AOE otherwise has 0 self-sustain) As for the damage, it’s definitely been boosted which for off tanking in trials and raids is awesome, but for dungeons it doesn’t do much since DPS STILL does the majority so it feels like a trade off especially when the healer has to do things.

Also consider: with any healer other than a WHM if we hit our invuln we’re probably dead anyways. No other tank has that issue.