The best uses for GTM for SEO goals are:
Event/Form Tracking – Tells you where your customers are in their journey and how they got to your site. Knowing this information will help you figure out how to close the deal and raise your conversion rate.
Metadata Injection – This gives you the ability to edit SEO features (page title, meta descriptions, etc…) in GTM.
Schema Implementation – Provides extra signals for Google to understand the content of your website. This improves your chances of showing up in featured snippets in Google search results.
Improved PageSpeed Score – If you implement your script resources through a GTM HTML tag, it will not negatively impact your PageSpeed Score. It could actually improve it.