The embed
Two lines, and the page is done with you.
A script tag and one custom element carrying an embed key. Everything the widget needs — which Microsoft 365 source, which layout, which colours — resolves from that key at boot, so changing any of it later never means editing the page again.
<script src="https://cdn.wpintegrate.com/v1/calendar.js"></script>
<wpi-calendar embed-key="your-embed-key"></wpi-calendar> cdn.wpintegrate.com stands in for the real bundle host.