Rolled plate is always hit or miss.
5/8″ is very thin.
The slot in the middle of the plate doesn’t help things.
Is this your own design or someone else’s? The first thing I would do, if I could, is thicken it up substantially. Based on the longest dimension, 24″ in this case, I’d want to start with either 1.5″ or 2″ thick plate.
If cost is no object, blanchard grind -> machine -> surface grind -> finish machine.