"When you buy something like DAS plinth you are buying weetbix wrapped in xmas paper."
That's a little harsh, no? I think most (maybe all) DAS plinths are 100% birch ply (not counting the veneer obviously) with the exception of the ones with rounded top corners that I mentioned above. Those have one layer of MDF on the top.
Look at these pics. These are all birch ply since they do not have large radiused top corners. Why would you call that a weetbix wrapped in xmas paper? Not trying to argue at all. Just trying to understand what you're saying. Seems like pretty solid construction to me.
https://i.ebayimg.com/images/g/LO4AAOSwnz9h20LM/s-l1600.webp
https://i.ebayimg.com/images/g/o-UAAOSwcVphkLUw/s-l1600.webp