I just started trying out Opera as well and turning off the X buttons was one of the first things I tried to do. This would be a nice feature. Firefox used to have this feature by default in about:config and there are now extensions that bring the feature back since they removed it. If there is an Opera extension to remove the X buttons then that would suffice.

When attempting to click an icon (e.g. WhatsApp) in the sidebar, my natural motion is to slam the cursor against the left edge of the screen in line with the icon and click (when window is maximized obviously). Unfortunately you actually have to have your mouse a few pixels off of the screen edge to be able to click the icons in the sidebar which requires waaaayyy more precision and slows you down.

Consider Fitts's Law when designing buttons like this: https://youtu.be/E3gS9tjACwU