Just to set the expectations...
To be honest, I don't think gmat preps are all that great either, but regardless, it's the best thing out there. However, I found my test day questions to be WAY harder than the actual test. Maybe I just got unlucky, and got the questions I didn't know how to solve. Maybe I was just extra nervous. I remember getting 4-5 sequence questions, which I hate, and I was like what the f, and then wanted to smash the screen
So good practice score does not necessarily translate to good test day score. I believe the gmat prep vs test day variance is +/-50 points, which I think is massive. Most of the time, it's on the MINUS side.
At the end of the day, your test day performance will depend on many factors, not just whether you know the material or not. Test day jitters, stomach aches, stupid sequence questions, lack of sleep from the night before, or whatever it may be. You can prepare as best you can, but there's just somethings that's out of your control, such as having diarrhea in the middle of the math section (definitely not a great start, but this happened to my friend)
Don't want to be negative, but just wanted to set some expectations.
Good luck with your studies!