I like the fact that you are aiming higher and not scared to attack difficult questions.
Quant forum has tonnes of material to target 600+,700+ questions. Before going there let me share my own thoughts.
1. The 20% questions you are getting wrong in OG. First make sure you note them in your error log
and master the approach to the solution. Your goal should be to master the OG 100% before moving to additional material.
2. Since you are scoring at a high rate. I advise you go straight to Advanced GMAT Quant
by Manhattan GMAT
. It is highly recommended on this forum.
3. For additional practice there are hard questions on this forum which are tagged as 600+ and 700+. The GMAT Club IOS/Android App makes it easy to find them.
4. Finally use GMAT Club Tests
they have a good collection of hard questions.
Hope this helps to point you in the right direction.
Cheers! You can do it!!
Give Kudos if you want to say thanks