You have a few options:
Schedule the meeting toward the end of your meeting load. That is, several weeks out. If you close cash from others, you don’t need to do the meeting. If you have not closed cash, then you may choose to be more open.
During the meeting, you can approach the agenda like a customer development meeting. That is, start off by saying I have a deck I can walk you though but before I do that I’d love to ask you a few questions to better understand your interest in us -- is that ok? After you get their consent, then you can ask them about their involvement with the competitor, how they are thinking about the space, etc. Based on their answers, when you start your deck you can decide how much you want to reveal. If you don’t feel comfortable sharing lots, don’t. You can just explain that your advisory board has asked you to limit what you can share. Or that there is a lot you are still working out. Or that given the conflict, you think it’s best not go into excessive detail.
NDAs are rarely signed and not likely a realistic option, just fyi.