There certainly things that you could sow directly. Annuals that readily reseed would work well such as corn flowers or cosmos. It is somewhat difficult to sow many perennials but again things that reseed would be good choices such as coneflowers and black eyed susan. It would really be better to go somewhere and purchase some small started perennials and plant them. You could then scatter some annuals into areas and it would fill in and bloom this year. Yes you will need to keep the mulch off until things are established and so you can water the seedlings if necessary. You do not have to wait until May to seed but if the seedlings are not cold hardy and it freezes you will be starting over. Here are a couple of links for you to look at and read through. Be sure to open all the links for the info. Look at the link for plants for 30 tough sights under the native plant link. Look for plants for full sun for your south side and part sun to shade for the east side. The clay makes it drain slower so look for things that tolerate moist soils. You really might consider adding some compost to the area to help your growing conditions. https://extension.umn.edu/find-plants/native-plants . https://extension.umn.edu/find-plants/native-plants. https://extension.umn.edu/lawns-and-landscapes/flowers-pollinators . Again be sure to open all the links for all of the info.