That sounds like a super cool project and I know people who would have so much more guidance for you! Increasing genetic diversity in tomatoes is so challenging. Because unless you start with a population that is already higher than the 3% crossing rate (ie promiscuous tomatoes), or you do manual crosses, you would have to start hundreds of seeds to find a few natural crosses. And there are already people breeding for this, and thinking about it obsessively (I’m not a tomato breeder), so I made an invitation for you https://landracegardening.discourse.group/invites/1u2J6gpT6c Sorry I can’t be more helpful 🙂 but it will be worth if for you to join (and is free).