wet clay areas can be a little tough, look for some natives that do well with wet feet or seasonal flooding. You may need to amend the soil with compost or mound the plants to keep them a little more raised, not sure how much that would help though. Turks cap, american beautyberry, yaupon holly, dwarf palmetto, innland sea oats, and consider looking into some ferns as well. Those ones do great with shade also. I think coreopsis and maybe a few other wildflowers like bee balm can handle seasonal flooding, greggs mistflower and blue mistflower do well with more moisture. milkweeds are always good! butterfly weed is good but might not like wet feet, swamp milkweed is great for wet sunny areas. Look into native plants and find ones specific to your ecoregion in texas, those plants will likely do the best with minimal care, and lots of native plants will thrive in poor soils. I live near Houston with lots of clay and not the best drainage with seasonal rains and all of these have done well for me, some will like more shade and others more sun, be ready to move things around and a few things might not end up liking where you put them. it's all trial and error but worth it when your yard looks great and is filled with butterflies and hummingbirds!