Quick comment on thickness - yes, with experience, 3mm is too thin. It's strong enough (the sections supporting the X rails are 100x50, the rest is 50x50) but is inclined to ring a bit. Thicker would provide more intrinsic damping, I think, and I would go for 4mm and, possibly, 5mm for some elements despite the extra weight. That's for a 1500x750 cutting area. There are other things I would change as well with experience, but at the time I built it, there was less information around than there is now.

But this one took me long enough to build so no plans for replacing it any time soon!