RE: Losing his eyesight gradually due to a rare disease, an introverted painter hurries to finish his last painting to participate in an important exhibition going trough an existential crisis.
What people are seeing is this.
1: You have a painter (Lead character)
2: Who is losing his eyesight (Specific problem)
3: Attempting to finish his painting before he completely loses his eyesight (Goal)
A great idea, I can visualize the story.
Also; even though I am not a painter I can understand the plight of the lead character. Great!
“…going through an existential crisis”
I don’t know what that looks like on screen.
1: Going blind isn’t existential
2: But if it’s not about going blind, You haven’t given us anything hints as to what the existential crisis is about.
It comes out of nowhere, we can’t visualize it and doesn’t give us any hints on how it will affect the plot
Conclusion: Leave the existential crisis in the story if it’s vital, but I would drop it from the logline (Unless the crisis can be summed up in two or three words)
However, that is just an opinion.