After spending most of the day trying to add chapters to my own mp3 file, I eventually found this web-based version that works perfectly: https://mp3chapters.github.io/

What’s good about it is it takes timestamps in the simple format that you see on YouTube: eg 00:01:45.00 Chaptername.

I tried with: ffmpeg, atomic parsley, mp4chaps, mp3splt and for one reason or another the result would fail to add chapters correctly. I’ve been on stackoverflow, blogs, DeepSeek and ChatGPT. A paid option I tested out was good but cost about EUR 50. In other words: you can save yourself some hours by just going to the link above.