Is there a way to send Apple proposals for usability improvements? I think this would be nice to have if there already isn't a way to send UI feedback.

For example, is there an e-mail address dedicated for that proposal, such as usability-improvement-proposal@apple.com or something similar?
Apple already has a mechanism for this.  It's called Apple Product Feedback.


  Apple strives to bring the best personal computing experience to
  students, educators, creative professionals and consumers around the
  world through its innovative hardware, software and Internet
  offerings. Apple welcomes your feedback on its products.


Apple will accept feedback on every product they make from their devices like the iPad, iPod, iPhone, Mac computers, dongles and peripherals, Apps, macOS, iOS, Watch, TV, iCloud, Pages/Numbers/Keynote, XCode, Music and Pay (not a comprehensive list).

If you have an improvement you would like to share, I encourage you to submit your feedback, suggestions and opinions to Apple.