You should be able to customize the comments you are inserting using Ooxml, just make sure to use the correct styling format to style the comments.
Change the following part of your Ooxml from this:
<w:comment xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main" w:id="0">
<w:sdtContent>
<w:p>
<w:r>
<w:t>' + commentMessage + '</w:t>
</w:r>
</w:p>
</w:sdtContent>
</w:comment>
to this:
<w:comment xmlns:w="http://schemas.openxmlformats.org/wordprocessingml/2006/main" w:id="0">
<w:sdtContent>
<w:p>
<w:r>
<w:t xml:space="preserve">This is a </w:t>
</w:r>
<w:r>
<w:rPr>
<w:b/>
<w:bCs/>
</w:rPr>
<w:t>customized</w:t>
</w:r>
<w:r>
<w:t xml:space="preserve"> comment.</w:t>
</w:r>
</w:p>
</w:sdtContent>
</w:comment>
Notice the word "customized" will now be bold in the comment.
If you want to add further formatting, like highlighting, here is another sample:
<w:r>
<w:t xml:space="preserve">This comment is </w:t>
</w:r>
<w:r>
<w:rPr>
<w:highlight w:val="yellow"/>
</w:rPr>
<w:t>highlighted</w:t>
</w:r>
You can learn more about Office Open XML styles here.
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