Hi everyone - hoping that someone with structural engineering know-how can weigh in.

I am aspiring to build a triangular, three post pergola over a hot tub, to provide some cover, as well as support of wisteria that I can’t manage to kill. However, I’m concerned about my lack of structural engineering knowledge, and I want to make sure that the span of the horizontal supports is sufficient. The spacing I would want/need is 8.5’ x 15 x 18. I’m planning the vertical post to be 6x6x8s in a post bracket, set in 3’ of concrete in a tube. The horizontal supports would be, I’m thinking, 8x2s. I live in Zone 8 (Portland, Oregon) and we don’t experience deep freezes although we do get a ton of rain in winter.

Is an 18’ span too big for this pergola? Should I use something thicker than 2” for the horizontal supports? Are the concrete post brackets appropriate for this design? Are there any simple ways to attach the horizontal supports to a triangular set of vertical 6x6s?

Pics as examples. Thanks!