Hi Karsten,
By "not editing it", do you mean just open the worksheet with Excel Online but not click Edit in browser? If yes, based on my test, just opening the worksheet in Excel Online as read mode doesn't have the right-click option. We need to click Edit in browser and then can use the right-click to get those options. It's not feasible to change the setting for this behavior.
(click Edit in Browser)
(now right-clicking will get those options)
We understand users may have the requirement to get the same option in read mode. In this situation, I suggest you submit the feedback via Excel Online Uservoice. This is the best way to let the related team hear your voice. Your feedback will help us perfect our products and services.
Thanks for your understanding.
Tim