Do you know all the details of those cases? This seems to be your frustration, that USCIS should process cases in a manner of FIFO, but we should understand that many cases are being processed in parallel, and not every case is the same. Maybe your local office is awaiting the receipt of your I751 package from whatever center it is at and the person who currently has your I751 case is out sick, or on family leave. I know in a perfect world, those cases should be dumped on someone else, but the federal bureaucracy is not perfect. I understand the frustration, but you have completed what you could to get yourself to the end of the process (submitting the I751, filing the N400 when your window opened, etc.). Just let the system work and be patient. There are many stories of folks filing the N400 with the I751 pending who get conditionally approved for the N400, but the I751 package is still unadjudicated, and they still have to wait for it to catch up. Patience is needed and hopefully your case will move soon.
Good Luck!