This is going to be great. Enjoy your incredibly detailed process.
Here is a pic of my setup for my OI.
Using an internal beam, the '1x2' plywood strips keep the stations at 90 degrees to the strongback. The concrete blocks keep the stands, lined with the string, from moving.
Like you, I believe a good foundation, free from defects and variations, will keep the finished product true.
I cut my strips on a 3hp tables saw, and then ran them thru a drum sander to remove all saw marks. Less sanding, easier to true.
Enjoy your build. Cheers.