I had the same problem. When I deleted all the configuration files and everything was working fine.
I investigated what could be the problem with my optimal performance settings for SD and I think I found the cause.
See if you have bGroupRenderAllShadows=1 in StarfieldCustom.ini. This item with the value 1 causes the shadows to flash in the character menu and after a while it freezes and restarts Steam on SD. Just set it to 0 or delete it. It helped me.