Everything you've done so far is just fine. Try scarlet sage. It blooms all summer in full sun and grows very well in large containers. Also try some of the new seed-grown ivy geraniums. Soil in outdoor containers needs changing much less often than soil in pots kept indoors.
Rain is relatively pure.
It contains few of the chemicals and salts found in our drinking water. Heavy rains leach away salts deposited by fertilizing and by watering with the garden hose. All you need to do is remove about an inch from the top of the soil in the pots and put in about 2 inches of fresh potting soil. Avoid potting soils containing vermiculite in outdoor plantings. Use Perlite, peat and sand.