Importing data from an old EE site
I have found that the best format for me is XML, it's easy to write and read. I've moved a number of EE2 sites to Craft by writing export templates using XML and importing into Craft with Feedme.
It's relatively simple, but can be tedious and time consuming as there are often tweaks needed either in the XML or the feedme settings.
I have a blog article I wrote a while ago that may help. Importing Entries into a Multi Lingual Craft CMS Website
I did not use the Smart Export Pro plugin but wrote my XML export templates by hand, I wasn't aware of that this existed.

I haven't used Content Elements but you say that it is similar to Craft Matrix so in that case I would look at the docs for importing into a Matrix field.https://docs.craftcms.com/feed-me/v4/guides/importing-into-matrix.html#setup-your-feed.
I have used this once to import into Craft 3 from a Craft 2 site with the code below. You should be able to use this as a model for your EE feed that you want to put into a Craft matrix. Below is the relevant bit from a larger feed.
    &lt;recollection_asset_type_matrix&gt;
      {% for block in entry.recollectionAssetType %}
        &lt;matrix_block&gt;
          {% switch block.type %}

            {# video #}
            {% case &quot;video&quot; %}
              &lt;subtitle_file&gt;{{ block.subtitleFile.first().url() }}&lt;/subtitle_file&gt;
              &lt;subtitle_label&gt;&lt;![CDATA[{{ block.subtitleLabel }}]]&gt;&lt;/subtitle_label&gt;
              &lt;vimeo_url&gt;&lt;![CDATA[{{ block.vimeoFilmURL }}]]&gt;&lt;/vimeo_url&gt;
              &lt;runtime&gt;&lt;![CDATA[{{ block.runTime }}]]&gt;&lt;/runtime&gt;
              &lt;poster_image&gt;{{ block.posterImage.first().url() }}&lt;/poster_image&gt;
            
            {# picture gallery #}
            {% case &quot;pictureGallery&quot; %}
              &lt;gallery_bg_image&gt;{{ block.backgroundImage.first().url() }}&lt;/gallery_bg_image&gt;
              &lt;gallery_audio_file&gt;{{ block.audioFile.first().url() }}&lt;/gallery_audio_file&gt;
              &lt;gallery_images&gt;
                {% for image in block.images %}
                  &lt;image&gt;{{ image.url() }}&lt;/image&gt;
                {% endfor %}
              &lt;/gallery_images&gt;
            
            {# memoir #}
            {% case &quot;memoir&quot; %}
              &lt;memoir_tagline&gt;&lt;![CDATA[{{ block.tagline }}]]&gt;&lt;/memoir_tagline&gt;
              &lt;memoir_bg_image&gt;{{ block.backgroundImage.first().url() }}&lt;/memoir_bg_image&gt;
              &lt;memoir_copy&gt;&lt;![CDATA[{{ block.copy }}]]&gt;&lt;/memoir_copy&gt;
              &lt;memoir_audio_file&gt;{{ block.audioFile.first().url() }}&lt;/memoir_audio_file&gt;

          {% endswitch %}
        &lt;/matrix_block&gt;
      {% endfor %}
    &lt;/recollection_asset_type_matrix&gt;

Unfortunately this import is long done so I am unable to take screenshots of the feedme settings.