All good advice, I would only lightly amend the soil if it's pure clay....I'm in Oklahoma, near Shawnee and Norman, where are you, and is the soil pure, red clay? Our soil can be clay in one spot and sand 6 feet away. If you want to try arbor vitae again, use a post hole digger and dig a "post hole" down as far as is reasonable in the bottom of the planting hole. Mound the soil somewhat so that the soil level currently at the stem of the plant is 2 to 3 inches higher than surrounding soil.
Arbor Vitae tolerate a range of soils, but I won't swear they'll thrive in pure clay. I'm rusty on soils for shrubs, but I seem to recall that some of the hollies will take clay soil with some care during establishment.
Pears, the fruiting kinds and ornamentals like Bradfords, are quite tolerant. Bald Cypress is tolerant of clay. Nandina will grow most anywhere. Even the Southern Red or Swamp Maple tolerates clay. It's too early in the morning for me to recall much more, but you do have choices both deciduous and evergreen.
Oh yeah, look into Red Tip Photinia, I've seen them thrive in pure clay.
Oh yeah...Tex above mentioned oaks, the
willow Oak and better yet the Water Oak are semi-evergreen, basically holding many green leaves all winter, and are clay tolerant.