Check out org-adapt-indentation. Setting this to nil should get what you are looking for. TAB will no longer indent past the heading text with this disabled.
On my Emacs, C-h v org-adapt-indentation:
org-adapt-indentation is a variable defined in ‘org.el’. Its value is t Documentation: Non-nil means adapt indentation to outline node level. When this variable is set, Org assumes that you write outlines by indenting text in each node to align with the headline (after the stars). The following issues are influenced by this variable: - The indentation is increased by one space in a demotion command, and decreased by one in a promotion command. However, in the latter case, if shifting some line in the entry body would alter document structure (e.g., insert a new headline), indentation is not changed at all. - Property drawers and planning information is inserted indented when this variable is set. When nil, they will not be indented. - TAB indents a line relative to current level. The lines below a headline will be indented when this variable is set. Note that this is all about true indentation, by adding and removing space characters. See also ‘org-indent.el’ which does level-dependent indentation in a virtual way, i.e. at display time in Emacs.