AnotherGMATtaker
Thanks for your message and wishes, Andrew! I am happy to share my perspective on the questions you have asked above -
-> Did you notice any difference between the online exam and the one administered at the center?
Yes! Apart from the logistical challenges of a home-based exam (I live in a multi-generational household, and it's rather complicated to take a 4 hours test with some form of activity happening around 24*7), I felt that the questions on the online exam were a bit more difficult (both for Quant & Verbal). I also felt that the questions were different from what I found in the OGs and official mocks. Not sure how much of an impact it had on my score, because I hadn't built a strong foundation for CR just yet and it did impact my Verbal performance. Again, this is just my experience, not sure if others in the same boat felt the same. I would still prefer the center-based exam over the online version any given day.
This view seems to fall in line with those from many other reports, namely that the online exam seems a little different and perhaps a bit more challenging. I can imagine that environmental factors would play a huge role, too. It can be quite hard to focus on an RC passage when so many other distractions are present.
AnotherGMATtaker
-> Did you comb through non-official or non-GMAT™ questions in an effort to refine your understanding or approach but ultimately decide that sticking to official material was best?
I actually started off with the OG questions - I thoroughly solved all questions, trying to self-analyze my weak spots and further refining them through a variety of sources (during the self-study phase). While it worked for most sections, I couldn't self-correct the course in CR. That's when I tried solving a bunch of questions from non-official sources (given I had exhausted all OG questions and mocks). I think the learning through these was only incremental.
That's when I decided to pivot - I started from scratch on
E-GMAT for CR and it was great (and the platform beautifully helped further optimize my prep for SC and RC)! I found the platform questions quite useful as well, to build the basic blocks and optimize preparation. Even the Sigma-X mocks were quite intuitive and helped me identify my weak spots in a full-fledged exam (which quarters I was faltering in, etc etc).
But then to round off my preparation, after putting my basics in place, I started solving questions from the advanced GMAT booklet (again, all my OG booklets and official mocks were done and I saw no point solving those Qs again). I wanted to be as close as I could to official questions to get into the groove again. I knew I could get a good score (I was aiming for 750+), and tried optimizing my pre-exam strategy to see this through.

It does not sound as though you blew through the OG in typical fashion, so I can see why you might have eventually performed at a higher level. Analysis takes time, and we have a tendency to chase more correct answers by practicing more questions rather than pause to reflect on how to arrive at accurate conclusions. It also takes a lot to admit that you can only go so far on your own. I actually think that what keeps most high-aspiring students, tutors, and test-prep professionals from acing the exam is not a lack of ability, but a lack of
humility. It is hard to learn something when you think you know it already.
AnotherGMATtaker
-> Finally, how about questions from the OG versus those on the mocks? Were they about the same, in your experience?
I think the official mock questions were a bit trickier than those on the OG (it could be the algorithm optimizing exam questions based on my performance real-time). The advanced OG booklet was a phenomenal resource (but I would only recommend it to people after they're clear on basic and have fair amount of practice).
The OG questions give a great flavor of what one could expect in the mock, AND in the real exam (if it's center-based). Again, I'll add the obvious caveat of this being my own view

Hope I was able to answer your questions. Have a great day.
I had a student tell me somewhat recently that he felt the official mocks were more difficult than the real exam, the first time I have heard this opinion. You seem to place the two on level footing, more or less. The OG might be somewhat easier, since the questions are not adjusted in the same manner as they are on a CAT. That makes sense. Good to hear your views on the Advanced guide, as well. This is the first time I think I have seen someone describe it as "phenomenal." Many high-scoring members mention the resource in their debriefs, so that in itself may be somewhat telling.
Thank you for taking the time to write such a thoughtful response. As I indicated in my earlier post, the community benefits by reading debriefs and responses from members who have achieved top scores.
Good luck with the next step in your journey.
- Andrew