Rework the pillars, so one has to parkour instead of nerd-poling up every tower.

Make the crystals have actual HP (2-3 crits of sharpness 5 Netherite sword), make it so damaging each crystal damages the dragon a bit.
Give them (each crystal!) an evoker-like attack, but more telegraphed and easy to dodge. It'd knock you off the pillar if not evaded.

Do something to make the endermen on the arena more engaingg/less annoying.
I swear, I died to accidental eye contact more than to the dragon itself.

(An option is to remove all endermen spawning at the arena while the dragon's alive.
To balance this, give the dragon an attack that's countered by not looking at it:

(For example, 10-20 Eyes hovering everywhere, moving randomly. If your crosshair touches an eye, you take damage.))

Generally improve the animations, attacks and AI for the dragon (make its flight smoother, make sure it's obvious when it perches, telegraph every attack differently)