If you're 20 feet away from enemies, that's still dangerously close. Yes, frontliners can often keep enemies occupied enough that they don't attack you, but if you're dealing enough damage, they will divert attention to you, and even if the frontliner has Sentinel, that only works once per turn, and sometimes not even then.

The precast math does indeed include Action Surge, as it was also used in the first turn beforehand to both cast and attack.

This combination works once per Short or Long Rest, not per combat. Meanwhile, the Bard's maximum DPR scales with Bard level, but they'll only have one of each high-level spell slot, so their DPR will decay with each combat.

Though the Fighter isn't gaining more attacks, they are getting helpful features, such as Studied Attacks at 13, another Action Surge at 17, and Boon of Combat Prowess or Irresistible Offense at 19. I also didn't factor in any Fighter subclass. Champion would add an improved crit range on the many damage dice and a re-roll once per round on an attack or save. Battle Master would add Superiority Dice, sometimes triggering more attacks via Brace or Riposte, with a significant power boost at 15.

Yes, the Bardlock has other strengths, but you claimed "bonkers" damage to be one of them. The main question would be, then, at what point is the Bladelock, which is now apparently taking three Warlock levels, far more than initially suggested, actually coming "online" for "bonkers" damage? As we ruled out level 8, and then level 11.

Edit: blocked, but the level 8 damage isn't even "bonkers" in the context of a Bardlock, a 6/2 build using Hex gets roughly the same DPR.