I was wondering what the best open source EPUB reader was, for both Android and Windows 10/11. It's ok if it's a different app for each platform.

I don't need to be able to convert EPUB to proprietary formats, I just need to be able to read some DRM-free EPUBs I have, preferably on an app that's open source so just does its job without collecting a bunch of data.

I have tried Calibre & SumatraPDF, I was so excited for Calibre until it never worked properly on the one textbook I needed. For example, whenever I went to the next page it would stall on loading forever, and this is apparently a known issue that's [according to the posts I read from the owner] caused by a graphics driver that I'm not interested in delving into just to read an ebook.

The ebook itself is pretty large with a lot of mathematical equations and images, but nothing my computer should be stalling on. The issue with SumatraPDF is that it can't seem to render the mathematical equations properly, and I couldn't find any simple way to load them without having to do more work.