I don't know about any applications that allow modifying character spacing but maybe Yomu Reader can make your life easier while you are looking for an option that does?
It definitely satisfies requirements 1, 3 and 5. The free version supports all the configuration options and you can test it for as long as you want if you don't need to have more than 10 books in your library at the same time.
https://apps.apple.com/us/app/yomu-e...62211012?mt=12
It's also possible that your eyes don't appreciate reading on a laptop screen in general. I can read on my ipad for hours without any issues but I never found a way to read on my macbook without eye strain, no matter which fonts or backgrounds I tried. Do you have an option to borrow an ipad somewhere to test if it's also the case for you?