Is it bad practice to disable (replace) the context menu?

I'm working on a web platform where users can select/highlight text and perform selection-specific actions on the highlighted text. The current implementation shows a custom context menu on selection (immediately - no right click required).

However, given that right-clicking is commonly associated with performing an action, could it be more appropriate to replace the default (OS-specific) context menu?

I understand that disabling standard shortcuts (e.g. copy/paste) will invariably lead to user frustration and is bad practice. I also found an answer stating that it depends on whether a feature is removed in the user's best interests - which I find rather vague. Thus, let me specify the question:

• Is it bad practice to change default UI behaviour, in casu to replace the browser context menu?
  • If so, what could be an acceptable alternative? Showing both menus simultaneously seems a bit crass.
  • If not, should the replacement menu mimic default behaviour (e.g. contain a "copy" button)?
• Is it important whether the replacement menu looks substantially different from the (expected) default menu?
• What about replacing the -webkit-touch-callout menu on iOS/Android? Do the same conclusions apply?