I think other than practicing your enunciation to be extra clear for the most important commands as well as devising easily parsable command phrases is about as good as you're going to get. It seems to me Voice Control is an OS level feature and as such apple's not going to allow you to do any tinkering. The ability to create your own phrases for commands should let you create command phases that can be said very clearly but not necessarily directly correlate to the action you're trying to perform. I'd also imagine it's using similar technogy to Siri so if you haven't taught it your voice through the 'hey siri' menu it might be worth a try.
It might be useful to have some examples of what kinds of commands you're trying to issue and having fail?