You can achieve this by formatting the NUMBER for Heading 1 to be 1pt size, hidden (white text as well so not visible with paragraph marks on, and followed by nothing). In the screenshot below, I have inserted the Number style "1,2,3..." but it is "invisible" in the "Enter formatting for number" textbox above because I have formatted the Font (button alongside) for the number to be 1pt, white and hidden text.
Leave the font size for the Heading 1 style itself (ie its TEXT) as is (eg 11 pt).
Then after you apply the Heading 1 style to your paragraph, type "Heading" or "Section" but use a STYLEREF field for Heading 1 style (which will find the current "hidden" number for your paragraph) and add a CARDTEXT switch (to change it from 1 to one).
Note also the text indent is set to 0 and follow with nothing.
Using a STYLEREF field also means that if you add/move main headings, if you select your entire document and press F9, the "text" numbers will update to their new value.
To avoid having to insert the STYLEREF field each time, you might find it easier to copy your first heading once it's set up with the field, and then just paste it for each new main heading.
and the field code (SHIFT F9 to toggle between code and result)