There is no risk here at all. Mutual masturbation does not transmit STIs.
There are no "what if" scenarios here that make it a risk. I can tell from your previous posts that anxiety is an issue for you. In October, CurfewX mentioned that it might be a good idea for you not to go to sex workers because you get great anxiety from it. That was good advice.
Here are some posts from our experts (when we had them) that may help -