To quickly add a Table of Contents to all your blog posts at once, install the Jump Links app: https://apps.shopify.com/toc
This is the only Shopify Approved App, which means it is trustworthy and compliant. No sketchy code from random websites or restrictions from a full theme.
The app is run on its own 3rd party AWS Server so it will not slow down your site.
It's managed in the US and has a full time customer service team, so any questions will get answered fast!
You can customize the look completely and even remove specific blog posts if you wish.
There is currently a 7 day Free Trial offered, so give it a try!