LinkedIn specifically has a section for project work that is different than the Experience section (resume). It is best not mix the two. Titles make a difference in terms of job seeking, but for little else. Recruiters and HR professionals search LinkedIn for specific keywords, titles, and triggers, so every word matters when you're trying to find a job. What's far more important is the accomplishments/achievements that are listed below each heading, with quantifiable measures. If you're not job hunting or trying to raise money for a new enterprise, then most of what you write on LinkedIn is just PR. Make the impression you want, but be authentic. You have to write something that you would actually say to people in an in-person conversation. If your LinkedIn story and your in-person story aren't the same, then people will not trust you anymore after experiencing the discord.
Titles don't mean much; but "lead" implies it. Linkedin descriptions aren't that important; people who hire want to see a detailed resume and that's where you can list all the things you did to take the project from scratch to completion. The older items on your resume should shed some of their details, because hiring people are biased towards more recent efforts.
You are still a co-founder. If you have founding partners, why worry about a silly title that may just piss off your partners by trying to over value yourself. Trust me I’ve been there. It’s not worth it if it will effect the relationship with your partners. Call yourself co-founder.