I use 1/2" diameter generic concrete steel rebar from Lowes in my garden. Work great. Have used same 36" to 120" rebar sections for 25 years. I cut rebar with a Sawzall. A manual hacksaw will cut 1/2" rebar too, more slowly.
Even UV coated fiberglass do not last that long. I would stick with steel, rebar, or vinyl coated steel, instead, unless you expect to run your tiller over the stakes.
Best tomato support I've ever used is a heavy duty metal cage shaped like a cone. I've made round cages out of field fence, but they don't nest together for storage in the winter like the cone shaped ones.
We use the sunguard 3/8 smooth fiberglass post for rotational grazing from TSC.
Sometimes they get hit and brake off the bottom 5" or so.
I cut them off and sharpen them on a belt sander and handle them without worry of splinters on the top section. We use these in the garden to mark rows and tie up plants all the time.
If the ground is soft and I have my carhartt on I just use my shoulder against the top while gripping and shoving down with bare hands to put them in the ground.
Otherwise we only use a rubber mallet to put them in the ground if its hard or frozen. Some of them have to be over a dozen years old now. Use a metal hammer one time and you will always be getting splinters.