I have a belt sander. I've been checking out the Dynafile. I don't see myself using one enough to justify the cost, but I have seen a few cheaper copies, and I might try one of them. Or not.

I doubt that I will go to the expense of a Kevlar saddle. I will probably lay an extra layer of glass in that area, as I'm not as concerned about the weight. I carry my Ride around and it weighs 67#, so a 32# SUP isn't going to be an issue. I am definitely going to embed all of the wood backing, and I'm curious as to why you are using Western Red Cedar for backing. I'm guessing it comes down to weight, and I think it is probably a good application. I have access to plenty of it, and I will probably us it, too. But I'm definitely going to use the Ipe or Cumaru for the stringers and maybe the outside edges, too.

I'm starting to get excited about this! Thanks for all of the info, and for setting the bar up there!

I use my Dynafile a lot. Don't know how I managed without one.
If not Kevlar/Graphite you need 8 oz or thicker glass for toughness on the deck. One layer of glass over foam is just too soft. But then you are going to need to add foot grip material. Because of the texture, Kevlar/Graphite precludes the requirement for foot grip material.
And I suggest white colorant for the entire top. Graphite powder in the epoxy on the bottom for slickness.
The Western Red Cedar was left over from my last kayak build. It is strong enough and light. Ideally you would use Teak, but most any wood will work.
I suggest US Composites for your composite components.
Looking forward to your build. Seems like you have some good ideas.