Muscadine trellis design

I'm in North Texas, zone 7D or 8 depending on who you believe. Dallas area, heavy black clay soil that's probably pH 8-ish.

We have about 100' of south-facing fence and 30-40' of west-facing fence in our back yard. Solid wood fence, built less than a year ago. I don't want things growing right on the fence (allegedly will make it rot out a lot sooner due to moisture)... but I want to use that vertical space!

I have a general layout for the back yard constructed, and am dedicating approximately 3' of space along the edges of that fence line to vines. I'd like to have muscadine grapes inter-planted with scarlet runner beans. The beans will help boost the soil quality (reducing fertilization needs) and attract pollinators, as well as being a second food crop in the same space.

The standard layout I'm seeing for grapes & muscadines seems to involve 4x4 or 6x6 treated wood posts, about 6' high (I can go a bit higher due to yard layout, but need to keep it in reach), with one, or at most, two wires strung between them for the vines to grow on, and one grape vine every 20' or so.

This is what everyone's doing it, so I'm sure there are good reasons to do so... buuuuuuut.... has anyone tried putting more wires on to get a denser (if potentially less productive per square foot of leaf structure)? Co-planting runner beans (or any other vines) like I'm doing?

This seems like it'd also be more likely to shade out/discourage grass and other such "weeds" from growing at the ground-to-one-foot level under the vines (there will be sun there). Or should I plan on a ground-cover type plant?

There has got to be a better, more multi-function & multi-crop method of doing grapes/muscadines than the mono-crop type approach that seems to be "the way". Any experience?

If this is the wrong forum, please move the post.

Having lived around a muscadine vineyard in GA, I think plain convenience may be the main reason for the "conventional" trellice design.....keeping the grapes at convenient height for picking while enabling a mower to get up under the canopy. In the wild muscadines go way up into the trees, so you could obviously lead them along any kind of support you want.
Scarlet runners are a bad choice for the South. They are a high-altitude plant in the wild and like cool summers, like England. If you want an annual legume vine, consider other pole beans, limas, vining cowpeas, or hyacinth beans..... You might also consider perennials, like groundnut.

My understanding was that scarlet runner beans are a perennial in the south. Could be wrong, my wife did the research on that one. Any suggestions for other perennial groundcovers or vines that would work well with grapes?
Groundnuts require digging to harvest - not a good choice with the grape vines.

I'm wondering if you came up with a good solution? I'm planning to re-try muscadines and would like to know if you found a better trellis design and a good nitrogen fixer for our Southern climate?

Around here, I see most "home growers" using a wooden structure -- like a square/rectangle box up on posts. The commercial growers use the single (or double..."T" ) wires... everyone keeps the ground free of other growth...

I'm using a 2-wire trellis (one wire at 30", one at 72") with a wood frame. My seed starting attempts were a total failure this year, so I wasn't able to get any hyssop growing under it. I probably got some of the pruning wrong, but at least half of the wire has vines on it.

I'm still experimenting but I had lots of vetch seed last year and put that under the vines. I can't really recommend it since I don't have prior experience to compare it to but my rabbits got plenty of vetch and the grapes produced well. My chain-link fence post trellis idea needs refinement since the posts were pulled together by all the weight of so many grapes. The latest ones have a top rail and a second wire pulling it in. The top rail uses the cast metal ends so I'm curious how they will handle the load. I've tried wooden posts on blackberry canes years ago and mine rotted out too quickly. That's why I'm going with the expensive metal.

I have approximately 20 vines of muscadines to move this winter. I am going to use cedar posts and 2x4 wire fence to build my new trellises. I use white clover as ground cover/nitrogen fixer and spread compost in the spring letting it settle into the clover.