I launch my boat (18 ft CC) by myself all the time under the I10 bridge in Seguin. I have also launched under the I10 bridge in New Braunfels but the concrete ramp is a little shorter and the ramp is subject to stronger currents. Both places have plenty of parking.
Good luck... I learned a lot about fishing the Guadalupe on this forum, particularly from the Banker-always fishing. You can bank on his recommendations (pun intended)