Folding JSON strings to send in header variables - Mailgun

I'm trying to use the JS Mailgun API to send emails. Have it working fine, until I throw template variables into 'h:X-Mailgun-Variables' like so, where jsonString is very large (17000+ characters):

const mailData = {
    from: 'Insights <insights@hello.net>',
    to: mailAddress,
    subject: `Insights: ${DAYS_OF_WEEK[date.getDay()]}, ${MONTHS[date.getMonth()]} ${ordinal_suffix_of(date.getDate())}  ${date.getFullYear()}`,
    template: "template1",
    'h:X-Mailgun-Variables': jsonString,
  };

Looking at the documentation here states the following:

Note The value of the “X-Mailgun-Variables” header must be valid JSON string, 
otherwise Mailgun won’t be able to parse it. If your X-Mailgun-Variables 
header exceeds 998 characters, you should use folding to spread the variables 
over multiple lines.

Referenced this post, which suggested I "fold" up the JSON by inserting CRLF characters at regular intervals. This led me here, which still does not work, though logging this does show regular line breaks and is compliant JSON:

const jsonString = JSON.stringify(templateVars).split('},').join('},\r \n');

Any insight into how to properly "fold" my JSON so I can use large template variables in my MailGun emails?

UPDATE:

As requested, adding my code. This works when data only has a few companies/posts, but when I have many companies each with many posts, I get the 400 error:

function dispatchEmails(data) {
  const DOMAIN = 'test.net';
  const mg = mailgun({apiKey: API_KEY, domain: DOMAIN});
  
  const templateVars = {
    date: theDate,
    previewText: 'preview',
    subject: 'subject',
    subhead: 'subhead',
    companies: data.companies.map(company => {
      return {
        url: company.url,
        totalParts: data.totalParts,
        currentPart: data.currentPart,
        companyData: {
          name: company.name,
          website: company.website,
          description: company.description
        },
        posts: _.map(company.news, item => {
          return {
            category: item.category,
            date: new Date(item.date),
            url: item.sourceUrl,
            title: item.title,
            source: item.publisherName,
            description: item.description,
          }
        })
      }
    })
  };

  const jsonString = JSON.stringify(templateVars).split('},').join('},\r \n');

  const mailData = {
    from: 'test@test.com',
    to: 'recipient@test.com',
    subject: 'subject',
    template: 'template',
    'h:X-Mailgun-Variables': jsonString
  };

  return mg.messages().send(mailData)
  .then(body => {
    return body;
  })
  .catch(err => {
    return {error: err};
  });
}

from Folding JSON strings to send in header variables - Mailgun