Improvising on an instrument isn’t hard once you have a level of technical fluency. You can play whatever notes you want in whatever order you want to play them, with whatever tone and attack. Scratching is more limited. The music in the recording is set already, it’s in the order that it’s in, and it sounds how it sounds. Any expressive moves you make have to make sense in relation to what’s happening in the recording.
If you are still unsure if your mix is ready for mastering, send it to the mastering engineer and ask their opinion. Any professional engineer will be able to help you catch potential issues in your sound and give you advice on how to address them within your mix to ensure best possible final results for your mastered song.