How would they build an aqua farm? They can only hunt (or fish)

Farming arose in response to scarcity. Dolphins live in abundance and only take what they need. Humans horde.

How could they paint? They can only nudge.

In the sand.

How could they do sign language? They can only squeak and flap.

Why do they need to? Are any dolphins deaf? Regardless we don't know they can't. Perhaps they can see many details in their physical movements than we think are relevant.

Farming allows focus on other activities such as tech development.

They can paint in the sand that easily disappears? I'm talking about permanent pieces of art tho.

The point of understanding dolphins is so that they communicate in true language. Now it is possible that they are, see youtube link above about understanding rat squeaks.

They are also unable to harness fire or combustion... arguably a prerequisite of civilization and the easiest way to acquire/use exosomatic energy.

Given that they're not destroying the planet -and themselves with it- I'd give them at least the benefit of doubt.

Humans didn't really destroy the planet until the industrial revolution.

Also, if there's too many dolphins, there's starvation. Storing food would see them through.