Hi Dutchy, screws attracting water wasn't really a consideration tbh because my intention has always been to apply a finish to the outside of the frame.  This will consist of
Wood Preserver (2 coats)
Preservative Primer (2 coats)
Biocidal wood hardener (promotes filler and finish adhesion) (1 coat)
Fill screwholes
End grain sealer on all end grain
Flexible undercoat  (1 coat)
Zinsser All Coat Gloss (2 coats)
Plus when these are installed they will go in with some exact fill expanding foam, which should waterproof everything.
I may have made them differently if I had room and a proper workshop, but in my little living room workshop, screws provided a good solution to the problem of how to assemble the frames. There is no hint of any movement in any of them. I was toying with adding some splines but I really don't think it's necessary. 

Doing the best I can with what I've got