Here is how I do it, just for comparison. I only use 4x4. but make sure the angles holes have concrete in the bottom so that the timber can have some footing to press against. I run rows up to about 120' with 3 T posts over the length.
If you decide that you don't want to trim roots, then you have to commit to digging a hole large enough to avoid J rooting.
I personally don't hesitate to trim roots, but i have only put 86 vines in the ground.
Aside from any root trimming "controversy"*, trimming roots takes far, far much less time than digging a much larger hole, then filling it in. My grafted grapes came in bundles of 50 (I have planted vines numbering in the four figure range). I just took a lopping shear trimmer to them. As a bundle. I ain't subtle. They did fine. Just saying. Grape vines are very sturdy. Don't helicopter parent them.
*To me there is no controversy, only people who understand biology and those that do not....
You will likely need more than you have at present. I'm a welder, so I built my trellis out of pipe, and set it four feet in the ground with four bags of sackcrete per end post and brace. I have one that is pulling the end post up out of the ground. Thought my deal was bullit pruff....guess not!