Of interest to this list might be the following book. Not a free or open-source one, unfortunately! But apparently having SageMath exercises is getting more popular.
https://www.crcpress.com/Introduction-to-Number-Theory-2nd-Edition/Vazzana-Garth/9781498717496"Provides calculations for computational experimentation using SageMath, a
free open-source mathematics software system, as well as Mathematica®
and Maple™, online via a robust, author-maintained website"
I've cc:ed the authors in case they haven't interacted much with the Sage community before and would like to join the sage-edu list. Also, I couldn't find the "robust" website, perhaps it's a CRC login website.