Here's the problem as I see it. You can't simply rework an existing job. There are expectations on how it's 'supposed' to work based on prior iterations. DRK's difficulty is that there are not one, but three DRK job designs, each of which is different in its own way.

Heavensward was based off of a single resource (MP) that decayed spontaneously over time. You could toggle this off during downtime by turning off your primary damage buff. At baseline, your standard combo generated MP at a rate such that your total MP would gently float upward. You made a decision at the end of every combo about whether you burnt this off or not. During Blood Weapon/Price windows, your MP rose more rapidly, requiring you to use Dark Arts Souleater more often. Dark Arts allowed you to use MP to toggle on special functions of your abilities for additional damage or utility. Aside from this, the job was largely a string of fast paced oGCDs and reactive procs related to either taking damage or parrying.

The main reason why it was revised was because it was possible to become resource starved if you mismanaged your MP, which could be dangerous if you messed up at the wrong time. Also, it's worth noting that while everyone looks back at Heavensward's Dark Arts system favorably, it would never work in the current era because we're much more focused on damage trade-offs. You would never use Dark Arts for utility if there's a potency price tag attached to MP.

Stormblood was based off a dual resource system. MP no longer decayed spontaneously over time, but also didn't tick up over time. You could reactivate your natural MP regeneration by turning off your primary damage buff. So at baseline, your MP generated a lot faster. Because there was no natural MP drain, the game forced you to constantly burn this off with Dark Arts uses. This was compounded by a lengthened Blood Weapon window under Delirium.

This resolved the issue of you becoming resource-starved and losing Darkside. In exchange, you were punctuating more GCDs with Dark Arts than Shatner does periods with words. It was a clunky, unwieldy system. However, it was extremely good if you lived next door to the servers, because everything was a double weave. It was changed because most people don't. It also killed off a lot of Heavensward's more fun abilities by turning them into either role actions, or locking them behind Grit.

What Stormblood did well was create interesting interactions between your two resources, especially in multi-target situations. TBN uses MP and gives you blood. Quietus and Delirium Blood Price use blood and give you MP (or both in the case of Blood Price). Abyssal Drain uses MP and gives you HP. You juggled resources to get whatever effect you wanted. Unfortunately, they didn't allow any of these shenanigans in single target, perhaps for good reason.

I think that if you want to rework the core gameplay loop, you have to pay homage to all three versions.

So how do we set up a two combo system that fulfills this? I think the easiest way to do this is to separate out MP and Blood into two different combos, and make this the basis of converting MP into Blood. Your baseline combo restores MP on each step. This is important because it ensures that you're never locked into a no MP state. You spend MP on Edge and Flood as normal, except this time to gain Dark Arts procs. You consume a Dark Arts proc to activate your second combo, which gives you Blood on each step. Get to 50 blood, use Delirium for your burst window (which we can talk about designing later). Once per minute, use Living Shadow to gain 50 blood for a burst window that's automatically synced up with raid buffs. That's how I'd see a two combo system working on DRK, rather than having a maintenance combo system. You'd have to similarly design the rest of your cooldowns to play off of exchanging MP for blood and vice versa.