Allowing collaborators to only edit certain columns



We've produced a snag/punch list on Smartsheet and the sheet is shared with the sub-contractors working on our construction site. Our 2 site managers have full admin rights to allow them to add snags and assign to the relevant contractors. We only want to allow our sub-contractors to add a comment to say the item has been actioned, or to add a photo to the 'completed photo' column. We have given them only 'commentor' rights, but they are still able to tick the checkbox to say the item is complete, we obviously don't want them marking their own items as done without us checking them first.

How do we allow/restrict access to certain columns please ?

Screenshot below of column headings which may help to explain