I've already read two translations of Meditations (MA), the Handbook (Epictetus' student), and On the Shortness of Life (Seneca).

There are patterns in the aforementioned books which I have understood, but the core of the philosophy is something I have not fully grasped yet, and it seems like Stoicism by John Sellars is making it super clear for me. It is the second book suggested for newcomers in the FAQ on this subreddit.

This is why I came to ask if it's a complete introduction to Stoicism as I do not want to dive deeper without fully understanding the core of it.

It also does not help that English is my second language, so some terms mean something completely different while reading the works of Epictetus and Seneca.

I also have the Daily Stoic by Ryan Holiday, and it seems like a cheap self-help book quoting Stoicism.