If you want to be involved in calibre development, then the calibre web page has a "Get Involved" section. Which is
https://calibre-ebook.com/get-involved. And the section of the forum you posted in is explicitly for discussing things related to the development.
But, I think Kovid was talking about your reference to the UI. There have been plenty of people popping up criticising the calibre UI and offering to "improve" it. All of those threads are in the forum to find. You can look at them and see what happened. Or you can make your suggestions. And if you can back it with code, then people can try it.
Personally, I don't know what is wrong with the interface. My experience with "modernized" or interface refreshes is that they tend not to be as good. Most have less information and make it harder for me to do what I want to do.