re: Making a form for an arch

I’d go a few at a time both because the force will be more manageable and because a lot of layers with the equivalent of ball bearings in between will make you think of taking up herding cats as an easier alternative.
As for thickness, I would make the mold about 3 1/2" and cut my pieces just less than that. Then I would screw a few sticks on the sides of the mold to control the side slip. You should end up with a little trim room for the inevitable misalignment you will get and still have your 3".

About using a strap….. I think the reel would get too full with the amount of distance you’d have to pull. I haven’t done that but ratchet straps really only work well if you can snug them up empty and only tighten the last bit.
Is that at all clear?

Stack three pieces with glue and put a tack through one end to keep them from sliding lengthwise, then clamp that end and work toward the other.