How to read data using the spreadsheets API without using oauth2

This is Yet another error 403 on Google Sheets API. Um trying to read the data from a Spreadsheet and expose it on a website, using some custom HTML filters.

So, there is this question: Error 403 on Google Sheets API

Still, I cannot figure out what to do. I have setup my service account in the google cloud platform, granted it permissions to access my Spreadsheet - I have even made my SpreadSheet accessible via link (but that is restricted to emails from my organization, so it seems useless anyway). I've also activated domain-wide delegation for my service account - didn't make any difference at all. I'm pretty sure the issue is not related to a wrong key or some typo because, when I remove the access to the service email from the sheet, I steel get a 403 error, but it comes with a message saying the caller doesn't have access to that spreadsheet.

People keep mentioning OAuth, but I don't want to use that, since I'm intending to use a simple API access, as in this Google GitHub example. I'm using my business account, so there might be some issue related to that.

Here is the HTML (which I'm running from a simple HTTP server on python, not directly, given the fact the GAPI doesn't handle localhost/ sources very well):

<!DOCTYPE html>
<html>
  <head>
    <title>Google Sheets API Quickstart</title>
    <meta charset="utf-8" />
  </head>
  <body>
    <p>Google Sheets API Quickstart</p>

    <!--Add buttons to initiate auth sequence and sign out-->
    <button id="authorize_button" style="display: none;">Authorize</button>
    <button id="signout_button" style="display: none;">Sign Out</button>

    <pre id="content" style="white-space: pre-wrap;"></pre>

    <script type="text/javascript">
      // Client ID and API key from the Developer Console
      var API_KEY = 'My-API-KEY';
      var DISCOVERY_DOCS = ["https://sheets.googleapis.com/$discovery/rest?version=v4"];
      var SCOPES = "https://www.googleapis.com/auth/spreadsheets.readonly";

      function handleClientLoad() {
        gapi.load('client', initClient);
      }

      function initClient() {
        gapi.client.init({
          apiKey: API_KEY,
          discoveryDocs: DISCOVERY_DOCS
        }).then(function(){listMajors();})
      }

      function appendPre(message) {
        var pre = document.getElementById('content');
        var textContent = document.createTextNode(message + '\n');
        pre.appendChild(textContent);
      }

      function listMajors() {
        gapi.client.sheets.spreadsheets.values.get({
          spreadsheetId: 'my-spreadsheet-id',
          range: 'Sheet1!A1:A10',
        }).then(function(response) {
          var range = response.result;
          if (range.values.length > 0) {
            appendPre('Name, Major:');
            for (i = 0; i < range.values.length; i++) {
              var row = range.values[i];
              // Print columns A and E, which correspond to indices 0 and 4.
              appendPre(row[0] + ', ' + row[4]);
            }
          } else {
            appendPre('No data found.');
          }
        }, function(response) {
          appendPre('Error: ' + response.result.error.message);
        });
      }
    </script>

    <script async defer src="https://apis.google.com/js/api.js"
      onload="this.onload=function(){};handleClientLoad()"
      onreadystatechange="if (this.readyState === lete') this.onload()">
    </script>
  </body>
</html>


Does anyone have anyclue on what I'm missing, or what else can I try?

Edit: sharing the spreadsheet publicly is not an option. The is a business email, and, therefore, the only option I've got here is sharing it domain wide, as I've already stated above. Also, this is a business spreadsheet, and therefofe contains sentitive data. Doesn't really make sense to expose it publicly. Besides that, I've already shared the spreadsheet with the service email. What I'm trying to achieve, after all, is to read the data from the spreadsheet without using oauth, and emdding it on a website (I know there is a built-in embed tool, but that doesn't suit me because I need to add a html filter).