Not open source, but mp3tag [1] and Notepad++ [2] (which is) would also be on my Windows list. For Mac I'd add MarkEdit [3], which is less than 3MB, carefully feature-scoped, and has a responsive developer behind it.

[1] https://www.mp3tag.de/en/

[2] https://notepad-plus-plus.org/

[3] https://github.com/MarkEdit-app/MarkEdit