Just came upon this thread and a few things caught my attention, at least based on the way we build up here, no idea how that applies in your area. Also, I have zero experience with buildings with roof purlins on edge, only flat.
Personally, I don't like the idea of site-built laminated posts. The joints, glued or not are more places for water to get in and rot to start. Under the same drainage conditions, a 6x6 post will always outlast a triple 2x6. There is a company in PA that factory makes them and I would use them in a heartbeat but difficult to get them shipped here. I paid nearly double for 8x8x16 than I would have paid for equivalent 22' laminated. Look up 'Rigid Ply Rafters'
I don't understand the design detail of 'not having to notch the post for the beam' with site laminated posts. The layers need to be perpendicular to the wall surface for the greatest strength. You wouldn't do your roof purlins @ 12' length by laying 3 layers of 2x6 on the flat. With a sharp chainsaw it won't take more than a minute to carefully cut each notch.
Load bearing scabs nailed to the side of a post is a poor substitute for making a proper notch.
IF you do decide to site-build your laminated posts, plan to have the shortest length of PT stop a foot above grade not at grade.
Don't forget that your time is worth something too. If it took me an hr to build a post to save 10 or 15 bucks, if buy the 6x6 and be done with it.
I wish I could afford to replace the falling down barn that I have for a proper shop but I'll have to settle for building the odd one for customers instead for now
I wish you the best with your project