I don’t care about Gehn’s issues, my point is that his ‘haphazard writing technique’ in his later years is mostly an informed inability. The Art is described as very difficult to master, yet he could manage to write habitable Ages by patching passages from other Books, something which should supposedly work badly, but not as badly as making up symbols or writing impossibilites. As you mentioned, his 5th Age was already good enough to hold for decades, despite being inorganic. Once stuck there, deprived of source materials, he was forced to come up with original descriptions. Even with his divinity complex (assuming that that has any influence on the Skill), this had to produce some kind of progress.
Besides, he was (for a short while) trained as a Bookmaker, not as a Writer, while Atrus and Katran had the assistance of Anna, who learned the Art before the Fall but apparently didn’t teach Gehn.
As a side note, since the Art doesn’t create Ages but merely links them, Gehn cannot logically be hold responsible for his Ages’ instability, only for his actions there. But then again, everything we know about him comes from Katran’s journals, which from a historiographic point of view should be considered biased.