If you are consistently scoring around 615 and your target is 685+, I would not make buying another set of mocks your next move. To add another 70 points to your score, you're going to have to improve your knowledge, skills, strategies, and techniques.
At this point, the more important question is not simply, “Which topics am I weak in?” Your mock analysis is already suggesting that there may not be one obvious recurring topic weakness. Instead, you need to figure out what is consistently going wrong in your problem-solving process.
Go through every question from your recent mocks that you got wrong, guessed on, or spent excessive time on and classify the reason:
1. Did I not know the underlying concept well enough?
2. Did I know the concept but fail to recognize when or how to apply it?
3. Did I have the right general approach but execute it incorrectly?
4. Did I get trapped between answer choices?
5. Did I spend too long searching for an approach that was not working?
6. Did I rush because I had already lost time earlier in the section?
7. Was this a genuine careless error, meaning I knew exactly what to do but made a simple execution mistake, or is there a deeper issue I need to fix?
That type of analysis can be much more useful than simply looking at a chart showing which topics you missed.
The fact that you are sometimes spending 4 minutes on a question is also important. That can indicate that a skill is not yet automatic enough, that you are not recognizing the most efficient approach quickly enough, or that you are staying with an unproductive approach for too long.
I would temporarily reduce the number of full-length mocks and return to targeted practice.
Rather than automatically choosing a handful of weak topics, initially target the failure modes you uncover. For example, if you discover that many of your misses come from not recognizing how to begin unfamiliar-looking questions, work specifically on recognition and approach selection. If certain concepts repeatedly cause those problems, then go back and strengthen those topics.
For Quant and DI, start with medium-level questions and work untimed until you can solve them accurately using a clear, repeatable process. Then gradually introduce harder questions and begin working toward test-day timing.
Do the same in Verbal. Since you have done less Verbal preparation than Quant and DI, make sure you are not overlooking potential gains there as well.
Also, solving all the OG questions does not necessarily mean you have mastered all the skills those questions test. The real test of mastery is whether you can see a new version of an underlying concept, recognize what is being tested, choose an effective approach, and execute it reliably.
A move from 615 to 685+ is significant, but it is certainly possible with the right work. Usually, the score gap comes from a collection of knowledge gaps, reasoning weaknesses, execution errors, and timing problems rather than one giant weakness.
Fix those systematically, and then return to the official mba.com practice exams to see whether the improvements are transferring to full-test conditions.
Good luck!