The best TOC plug-in is SimpleTOC!

Simple, effective, and very well supported. Other toc plugins seem bloated with features and code that really complicates something that should simply fit right into your theme styles without adjustment.
This plug-in is perfect for a cohesive fit into your site as it inherits your theme styles. It’s also compatible with the rank math SEO plug-in.

Supports reusable blocks too! Finds the headers buried in groups and saved as reusable blocks.
Excellent feature!

The best feature though is the outstanding support from Marc the developer. He responds and communicates well to all requests. That means a lot when you stumble upon a needed feature or request an improvement appropriate to the simplicity designed into this plug-in.

Five stars should be ten for this plug-in!

Kudos Marc for all you do!