A lot of websites owners focus too much on online marketing optimisation, but not on conversion optimisation — they bring the horse to water, but they don’t know how to make it drink.

Your usability principles are a good start. Some others I’d add are:
– Make use of familiar conventions. Users expect the logo to be top left, search bar top right, etc. Move things around and you just make things difficult.

– Employ the user’s own language. Don’t use jargon or corporate speak. Use terms the user can understand.

– Always test your designs on real users. Only by conducting usability testing can you find out what your users are really experiencing.