leftmargin length not working in enumerate environment

I need to write down arguments that are made up of numbered claims and where the conclusion is marked by a conclusion symbol. I have no problem modifying the way enumerate labels are displayed to have that result.

Unfortunately, these labels quickly get so wide that they hang over into the left margin. I’d like to indent the enumerate environment on the left so that the labels and the text are further to the right. Looking at a list of lengths in list environments (e.g., in this answer), I figured leftmargin would be the right length for me to manipulate. Unfortunately, it doesn’t have the expected effect of indenting the text associated with an item. I can use itemindent to indent the label, but that only indents the first line, not the main body.

A couple of constraints on answers.

• I’m aware of the enumerate package and for various reasons do not want to use it.
• I only want some, but not all, enumerate environments to be indented in this way. So a global change, e.g., by including AtBeginDocument{addtolength{leftmargini}{3em}} won’t work for me.

Here’s an MWE I’m working with, and a screenshot of the result. I’m using lipsum as an easy way to generate multiline text, and I’m calling amssymb to get me the "conclusion" symbol of the three dots.

documentclass{article}

usepackage{lipsum}
usepackage{amssymb}
begin{document}

lipsum[1]

begin{enumerate}
  renewcommand{theenumi}{roman{enumi}}%
  renewcommand{labelenumi}{(theenumi)}%
  addtolength{leftmargin}{5em}
  addtolength{itemindent}{3em}
item lipsum[4]
item lipsum[4]%
  renewcommand{labelenumi}{$therefore$(theenumi)}%
item lipsum[4]
end{enumerate}
end{document}

documentclass{article}
usepackage{lipsum}

usepackage{amssymb}
usepackage{enumitem}
setlist[enumerate]{label=(roman*),
                    itemindent=3em,
                    leftmargin=2em, % leftmargin=* if you like to have left margin as in main text
                    }

begin{document}
lipsum[11]

begin{enumerate}
item   lipsum[4]
item   lipsum[4]%
stepcounter{enumi}          % <---
item[$therefore$theenumi] % <---
        lipsum[4]
end{enumerate}
end{document}

documentclass{article}
usepackage{showframe}
renewcommand{ShowFrameLinethickness}{0.4pt}
usepackage{enumitem}
usepackage{lipsum}
usepackage{amssymb}

begin{document}

lipsum[1]

begin{enumerate}[labelwidth=2.5em, leftmargin=!]
item lipsum[4]
item lipsum[4]%
item[$therefore$ stepcounter{enumi}(theenumi)] lipsum[4]
end{enumerate}

end{document}