Ron, I have the Italian version of that Garniga set that allows for thicker doors. I think the US version goes down to 19mm thick typical for US construction. I have additional grovers for my set that you should be able to order. I have 6 and 7 mm in addition to the standard 5mm.
Myself I don¡¯t care to use 1/4¡± ply for any flat panel doors. Just seems flimsy. In solid I use 12mm thick and rebate or cove the back. For paint grade I use 1/2¡± MDJ and treat the same. For me groove thickness is not critical building this way.
Good to know about the bevel being available for this set. What I like about this set is you can do true tenons if desired.
I demo the Rangate bevel shaker set in the Alpine workshops. It¡¯s a nice precise fitting set but does not interchange with the old Garniga sets.
