Linear bearing upgrade options?

[Jonathan]

all the ball screws are 1605 from linearmotionbearings and they seam accurate but I can get up to 0.09mm of "backlash" or flex when I push and pull on the machine

If you actually mean backlash, then 0.09mm is a sign of the ballscrews being damaged, however it sounds like you mean flex in the machine? If that's the case then better rails is one way to help with that.

I am thinking of upgrading to a branded 20mm screw which should give me a bit more rigidity.

You don't need a larger diameter screw - it will only slow things down. The load rating and stiffness of 16mm ballscrews is plenty for the size of your machine.