"I do not paint enough."
Translation:Nem festek eleget.
Elég is not a noun, but I suppose you can look at it that way if it helps. It's quite often used in its declined forms, which are translated to the same English words, so I understand why it can be difficult.
If the subject does (or doesn't do) something often enough, or spends (or doesn't spend) sufficient time doing something, use the accusative form eleget, like in Duo's sentence here.
- András nem tanul eleget, nem fog átmenni a vizsgáin. -- András doesn't study enough, he won't pass his exams.
- Eleget gyakoroltam, így megnyertem a versenyt. -- I had practiced enough, so I won the competition.
Eléggé as an adverb can mean "in a sufficient way", or often it can be translated as "quite".
- A feleségem nem szeret eléggé. -- My wife doesn't love me enough.
- Eléggé haragszik ránk. -- He's quite angry with us.
Elég and eléggé are interchangeable and are translated to "quite" when they refer to a following adjective or adverb:
- Ez a hegy elég(gé) magas. -- This mountain is quite high.
- Elég(gé) rosszul érzem magam. -- I feel quite bad/sick.
As a numeral, elég stays in its nominative form.
- Nincs elég liszt otthon. -- There's not enough flour at home.
- Elég hibát vétettem már. -- I've already made enough mistakes.
If elég is referred to by a verb, it tends to take the suffix required by that verb.
- Eleget láttam. -- I saw enough. (lát valamit)
- Az erőfeszítései elégnek bizonyultak. -- His efforts proved to be enough. (bizonyul valaminek)
And with a possessive suffix, "elege van (valamiből)" can mean "to have had enough (of something)".
- Elegünk van belőled. -- We've had enough of you.
- Elegem van ebből az egészből. -- I've had enough of all of this.
This comment ended up a lot longer than I wanted, but I hope it will be helpful at least.
Your reasoning is fair, let me just say "Nem eleget festek" emphasizes quite strongly that "I do paint in the first place (... and it's just not enough)" Okay, you paint "not enough", how much do you paint then, with a positive part of speech? Why don't you say it clearly that you paint too little? Why make it more complicated with negation...
It sounds pretty much like "Enough? I don't paint (such/that)..." Putting it first makes it the topic and it doesn't work well as the topic. Usually you make introduced or well known things the topic, "elég" isn't even really standalone here, more like just a part of the "nem eleget"/"not enough" structure.
(On the other hand, "Eleget festek" could work because in that sentence, "eleget" can be the focus or something that closely belongs to the verb anyway. Adding "nem" between "eleget" and "festek" kills this interpretation.)