I found a cheap one that was ground to a very good point so I ordered a bunch more.

Those were ground the same way, but missing the centre. Result: Flat bottom.

If you want to do 18 mm ply, you'd need a 36 mm bit. That is BIIIIIG and needs a big and powerfull spindle.
Or you'd be doing minute increments, like 0.1mm per pass as you get deeper.