Best thing you can do is proxy pass a path of your website to some CMS, however when your CMS will have to execute CTAs on your SPA, good luck with that.

Worst thing you can do is allow HTML fields in your Headless CMS and pass it through in your SPA but it is like going back to 90s way of doing content.

SPA is not a good case for this type of website. What you need is WordPress. This problem cannot get solved with Headless CMS.