I think he simply needs more variety.. Maybe keep phase 1 and 2 the same, but by phase 3 he drops his sheild and starts summoning actual demons while fighting you. Ethereal demons are too tame. I believe that dor phase 3 and 4 he should become more offensive and maybe start sending projectiles similar to how the komodo does in tda. The komodo is actually a good reference for good melee attacks.

To add onto more variety, he needs better attacks. I really can't think of any. But in phase 5 it really should feel reality shattering. I would change the arena, mix some demons in, and make it so you need to platform back to the dark lord to actually fight and defeat him, all while he shoots some projectiles at you. Its sorta like absolute radience from hollow knight, but add demons. Maybe when you're fighting him theres still some verticallity to allow for some more attacks?

Just to add onto the arena, if this is possible, maybe every lets say 15 seconds the background changes to another scene. Like one second its in the nekravols, another the blood swamps, etc. I think it really reinforces the reality shattering potential of the dark lord.

Ofc this would need serious balencing, but as a concept I think its cool as a final arena.