I built my shed about the same dimensions 15 yrs ago. I 2x4 preframed the base with inside joists 16”” on center with blocking in between. I set the base on concrete pavers on the perimeter and some on the interior for additional support with 4-6” gravel tamped under each. Leveling pavers and setting base took some time to get it right bc my yard was sloped but not too bad. Still standing and solid. Live in MD