First, I'll say +1 to the other commenters here encouraging you to develop the idea on your own first before trying to bring in other people.
But let's say you're going to ignore all of that and still try to find a partner first. Your advertisement gives me zero confidence that you have an actual business here. Your idea is: a personal finance budgeting tool, but "fun and easy to use" and based on how you personally manage your finances. I have so many questions! And as a former Product Manager you should have answers:
• What specifically is wrong with existing personal finance tools on the market? What is the killer feature that makes yours better? Have you tried https://lunchmoney.app/ ?
• Let's see those wireframes! User journeys! Something! Right now I'm left with the assumption that you've sketched out some login dialogs and a transaction list but haven't thought beyond that.
• Where's your user research? If you're looking for a cofounder, that implies that you have a thing that needs to be built. Do you know what that thing is? Have you validated with real people that this is a real problem they have? How many user interviews have you done? Again, my assumption is that you've done none of this.
• What will be your role in the business? How do you propose that you earn your 50% of the equity? Remember that "idea guy" is worth nothing... you need to bring sales chops, key relationships / industry credibility, recruiting ability, investment money, or some combination thereof.
• If you've actually thought through this stuff and don't want to reveal your secrets, why not? No one is going to copy you (probably), and you'll get key insight into your market. Before you can ever sell this to a consumer you need to sell it to your early partners.
Finally, remember that some of the best cofounder partnerships work because the people involved have prior history working together professionally. Which of your former co-workers could you convince to quit their job and join you? If none of them, why not? What would you need to do to strengthen your pitch? Do that.