However, their fruits are mediocre at best. As you can see they occupy too much space. Should I dig them up? That would be a hard job. Or should I try some grafting? How difficult is it to graft grape vines?
Or are there better options other than digging up and grafting?
Second question is, I took some grape vine cuttings in winter. They start pushing leave now. But as you can see I put them a bit too close together. Should I dig them up and re-plant them in individual pots now or should I wait until this winter?
Do the vines produce poor fruit all the time, or just in their overgrown state? I realize you may not know the answer, but maybe try to let only a couple shoots produce fruit this year and prune out the rest? It’s possible the vine is so overgrown that it’s shading the fruit and not allowing it reach full potential?
The arbor could be problematic too. Here in the Mid-Atlanic wine grape growers do “leaf pulling” throughout the season to make sure the leaves don’t shade the grapes and they get direct sunlight. Some of that is to ensure the grapes are exposed to the wind and sun to allow them to dry out quickly after a rain, but I think some of it is to get them to ripen up well.
If those photos are recent it is not too late although I am no expert. Being in zone 4 I leave 5 bud canes, wait until the terminal buds start swelling and then cut back to 2 buds. This retards those 2 buds opening by 1-2 weeks as opposed if I left the 5 buds. This potentially helps me avoid damage from late spring frosts.
Be aware that your cut ends will weep liquid for several days. When it drys you will see a white, crystal like powder. Nothing to worry about.
Disclaimer. This is what I read and do for my zone 4 climate with low disease pressure. If you have major fungus/bacterial problems or a warmer climate you may need to do more research on the subject.
I don’t think it’s too late if those pics are recent.
Around here we grow grapes on a trellis with a low wire (3 feet off the ground) and train them in the Vertical Shoot Positioning (VSP) style. What that means in one cane per side of the vine is kept from the previous year and bent down and tied to the lowest trellis wire. Shoots grow up from the cane and they are spaced about 6 inches apart and each shoot produces one cluster of grapes. The shoots will need to be tied to something for support. I have other trellis wires that I time them to, but maybe you can attach rope or wire to the top of the arbor and let them climb that.
Your first photo has a couple good contenders for VSP positioning. You can bend a couple of those branches down (one per side) and attach them to something for support and then let the shoots grow up towards the trellis (with support).
All the stuff above the canes you are going to bend down (and attach to the trellis wire, if the was one will get pruned off and discarded. That will keep the grapes low where you can easily access them and also open up the top to let the sunlight down to the grapes