Can I change keyboard shortcuts? Or change mouse behavior? I prefer hovered button in right-click menu to activate on right mouse release, without an additional left click if possible. Also a hotkey/button to copy image to clipboard would be much appreciated

(Whoops, had this open in a tab and forgot to post)

There's only one popup menu mode, having activation on release would probably need a different design. For example, the menu opens with "toggle zoom" directly under the cursor, so that right-click left-click always zooms in and out, but it wouldn't be good if pressing and releasing RMB toggled zoom. The dropdown menus would have to behave differently than other items too (more options, bookmark tags, artist). It's probably different enough that it's better to stick to one mode (it's easier to keep things polished when I'm actively using everything in the UI, rather than alternative UIs that I'm not using myself)--but I'll think about it some more.

I don't think there's any way to copy images, since Pixiv doesn't implement CORS for images and the site isn't able to access them other than displaying them as images. You can shift-right-click the image to open the browser context menu to get access to the browser's "copy image", though.