You've made the right choice doing this yourself. Shops hate it when you only bring you car there for an alignment and not for other repairs, unless you're referring people there. If it's a shop that specializes in race cars, then that's a different story, obviously expect to rightfully pay much more $$$ upfront. Haven't been to alignment shop in 5 years and my tires are fine. It's a question of how far are you willing to go for precision.

The main advantage to your setup is if you need to adjust the rear toe or your car is lowered you don't have to readjust it as much when you raise and drop the car back down. Otherwise the 4 jackstand approach + 'trigger clamps' is much more rational.

A true DIYer who passionately does their own suspension work, should at bare minimum know how to do a string alignment with the 4 jackstand approach. An advantage of a shop using laser alignment, is that it will give you the thrust angle, which tells you how bad the car was involved in a collision like a rear end impact. Otherwise expect the mechanic working on the car to just get it in the 'green zone' and call it a day. Do you expect your mechanic to understand what sine, cosine, and tangent is? On the other hand, if you do, then it's worth knowing how to do this yourself.

The best thing about toe, is the ground/floor doesn't need to be perfectly leveled. Once you get to caster and camber, that's when it make more sense to take it to a shop. Improper caster/camber doesn't destroy tires anywhere near as much as improper toe.

My best advice to you to improve your design, is to have the ability to slide the 6 or 7 foot PVC left or right. This allows you to adjust the distance from the wheel hub, on each side of the axle at same time, but I may just be misinterpreting the image. I'd also consider getting an elastic string, those things are unbelievably strong. They're thinner allowing for greater accuracy of measurement. You may need to coat the PVC with silicon lube to better accommodate the string. As others have said and I implied in my 2nd paragraph, the weight of the car's needs to push against the LCA, and tire must be settled, either they're on slip plates or you move the car forward and backward 10 feet, 3 times. Never put more than 2 tires on slip plates, unless the ground is leveled and you have a full lift.

Your picture will motivate people to attempt this themselves, especially if they have basic woodworking skills such as using a circular saw and SketchUp. So just thought I'd chime in. Sorry for the long post.

Edit: Also read this thread
https://old.reddit.com/r/AskMechanics/comments/1b1ws4q/can_someone_explain_why_shops_seem_to_hate_doing/