Yeah I think from an aesthetic standpoint fastening the boards perpendicular to the long span is preferable and also makes the structure stronger (though you’ve got plenty of blocking).

Also a good point about the 6’ off cuts which are more useful than shorter off cuts.

Gm

I think you have to got longways with the way it is framed. Trex is supposed to be supported every 16".

What the hell are you planning to put on there? No kill like overkill right?

The simplest reason to run the boards width-ways (10-foot lengths) is:

Cleaner Look: You get a perfectly seamless walking surface with no messy staggered joints required along the 32-foot length.

Smarter Installation: It's easier and faster to install, as every board is the same 10-foot length.

Maximum Value: You get highly usable 6-foot cut-offs from your 16-foot Trex boards for your floating extension, saving you material costs.

It is the most practical and visually clean option for your dock.

I hope this helps...