It may be impossible to remain amicable once you lawyer up. Using "I saw a lawyer" or "I am seeing a lawyer" as a weapon won't help you.
Have you thought about a formal external mediator? Doesn't have to be a lawyer, could be interesting to facilitate the difference between you.
People want different things in life. Sometimes the differences are irreconcilable.
Do you have VC funding who you could discuss this with? Do you have a mentor? You probably need somebody in the business space who can advise you f2f not randoms online.
Not me. I tried to help a friend (at his request) in a situation around IPR and I fucked it up, he isn't a friend any more and nor is the other party. Don't want to be that guy ever again.