From what I've heard, their strategy has been to stall at being 6 months away from a bomb. They could've finished their program a while ago if they wanted to, North Korea did it with far less resources. They don't do it because:

1. It puts them in the dangerous MAD nuclear game theory with Israel
2. It would make them into a pariah state like North Korea and close off any possibility of sanctions relief.

They still want to be somewhat ready though for situations like this where full scale war with Israel is inevitable so if they can survive the war for 6 months they can do a test, establish nuclear sovereignty and get Israel to back off.