Welcome to the SeeMeCNC Community

From day one I have always had some issues trying to get my Onyx bed to be level. The great bunch of people at Seemecnc even sent me a new one to try because of my issues. Unfortunately I could never get it just right. No mater what when my bed would heat up the center would rise almost 1mm which when added to the Onyxes already convex shape made even the glass deflect. This was a great source of frustration on large prints but I love my Max so much I just said I will figure it out later...... Well later was tonight..... It hit me like a ton of bricks to use a piece of ABS and lay it around the outer most edge of the bed. This way when the bed heats up the outer and inner portions will finally be close. To my surprise it is a perfect fit, to the naked eye that is..... I hope this helps anyone else out there. I know this will cause issues with heat transfer along the edge, but the Onyx was already lacking in that area anyway.

A unique way to shim the Onyx. My concern would be the ABS "shim" could soften with the higher bed temperatures.
A piece of rubber like they use on window screens may work better and would not be susceptible to softening.
I hope this works for you long term.

I had much the same problem, I ended up heating it to temp and then using washers as shim under the low side.