I grow about 600 determinate toms; about every 6 feet I set a rough "tree" post that stands above ground +/- 6 ft. high, at both ends of my rows I have strong "mother" posts, these hold the tension so are angle braced with a separate diagonal post into the ground; heavy galvanized wire strands are strung parallel to the ground in spaced rows that let me tie up the toms as they grow; the wire is affixed into each post along the route at about a hammer length one above the other using U shaped pointed "staples" for barbed wire; the sag after every crop & gaps from shed bark require re-tensioning & re-stapling each of the wires (pull at the mother post).
PVC free standing structural strength is minimal, so you'd need to frame it &
this makes large scale growing costly.
Galvanized wire is sold in 100 weight rolls cheaper.

I use 7' T-post fence stakes. these are much stronger and will last 25x times longer than PVC (I have stakes that are over 110 years old and still strong. PVC as it is not meant to be outdoors will at best last 4 to 5 years before getting too weak and than having to be landfilled). Not to mention all the pollution created by the production of PVC (it takes some the most dangerous, toxic and persistent chemicals to create PVC).

I would think PVC would be too floppy and weak to hold up plants that will weigh over 250 pounds when mature and loaded with fruit. I know all but the stoutest bamboo is.

And there is a though, why not Bamboo-it is about as strong, will likely last a season or two longer than PVC and it is compostable and took no nasty poisons to create.

I grow around 600 to 1000 tomatoes a year, both determinant and indeterminant, mainly heirlooms

I am all about being frugal or cheap. I used untreated 2 by 4's that I ripped into 2 pieces. I drove them into the ground about 1.5-2 feet. They lasted great all season. I picked up many of them at constructions sites. Friends and Family member either built a house or build houses. I also used 1 by 4 ripped into 1.25-1.5 wide pieces and used them the same way.

I also use steel T posts at the ends and one in the middle of my 50 foot rows.

As far as string, I use polypropolene bailing twine, not the sisal type. The kind that you put on big round bales. The stuff has a UV protectant in it. You can buy huge balls of it very reasonable. I reused it from my parents farm. It is just trash to them and perfect for me!

Just an Idea