Will they stay somewhat open or will they start to fill in and look like a hedge as I keep them clipped to 6 feet in height? ==>> your problem.. is defining PRUNING ... if you SHEAR them.. give them a hair cut.. they will become the 'hedge' you dont want... if you learn to selectively PRUNE them.. you can achieve ANY shape or form you want ... e.g. topiary, at an extreme .... as to foundation planting.. IMHO ... nothing is planted ON THE FOUNDATION .. ever!!! first.. there should be 3 to 4 feet of clear space at the house ... for window cleaning.. ladders.. gutter cleaning.. xmas light hanging.. painting .. etc ... and given that you want a 4 foot plant.. on center.. the 4 foot above.. plus the 2 feet on center.. would mean you should consider planting these ... SIX FEET FROM THE HOUSE ... and ipso presto bingo bango.. no snow load issue ... so you would start.. by building a new front garden bed... to alleviate your concerns about 'such a large gap between the house and plants.' your descriptive words are fine and dandy.. but if you want real help .. you need to post some pix.. so we can 'see' it all.. and offer further suggestions ... oh.. and as to the grade issue ... a nice wall of retaining brick ... out 10 feet from the house ... backfilled with appropriate soil ... will make the foundation disappear.. and give you a nice bed.. to plant things in .. do plan.. much beyond these few shrubs.. its time to make an impact statement ... after all.. it is the front of your house. .. see pix below ... so much for the budget.. good luck ken my house is also on a hill ... here are some pix of the walls i build.. not very hard in my sand.. but heavy labor for sure ... if you look back by the hot tub.. you can see the basement wall ... on this one.. look deep into the pic at the wall.. note nothing is planting NEXT to the house .. otherwise in the forefront .... the only way to properly prune a purple plum.. lol ...

Peonies can make a foundation planting depending on cultivar chosen. Start visiting the botanic gardens in the Chicago area and really look at the foliage. Some make good plants for what you have in mind. The colors vary depending on parentage. My foliage ranges from a sickly yellow green large leafed type to a darker medium leafed color. You would not want a true tenifolia because sometimes they go dormant too soon. Two problems with using peonies as a foundation plantings are the soil may be very alkline, must have tested before planting, and herbaceous peonies die back to the ground and would not provide any winter coverage. After the blooms are past you would need to cut the bloom stems off below the surface of the foliage so that the plants form a nicer shape.

Do you have any inspiration pictures? This is the time to do hardscaping so you should try to decide. You have a back porch, will it hold the whole family? Will there be lawn in the back yard? Where will cars park? Is there a driveway? I think you should leave an open area ( no hardscape and no trees) closer to the street for more parking space in the future. You may not have an RV now but who knows what the future holds? I do mean to have the area planted, just with things easier to remove if you ever decide. I think you should get some graph paper and draw your plot in a to scale drawing. Then you could try too place plants and other elements by drawing onto the paper. Some people make their mock up with their computer. I don't know what software they use. Even if you do not think you are creative, just doodle at first. When you have something post it.

It's often more efficient to go and see what is available to you at local outlets first and then make your choices from among those you like the look of. Unless you are planning to order the plants in from elsewhere. Otherwise this looks better written than usual. Only a small number of evergreen kinds but all the shrubs included are supposed to be reliable. So it could still help you zero in on something that will work out: Oklahoma Proven: Plant Selections for Oklahoma | Oklahoma State University (okstate.edu)