I think its more that once Chara found out that the barrier could only be crossed with the help of a monster's soul, and the fact that only a boss monster's soul persists long enough for a human to use it, they probably came up with their plan in order to keep from any of their new family from being sacrificed in order to let them through the barrier. And even if they go through, there was no guarantee they could convince the other humans to break the barrier without some potential.. coercion.
I see your point, but again, I really think the buttercup incident had a huge negative impact on Chara. Honestly the way boss monster’s lifespans work Chara, if they had wanted to, could have lived far into their adult years while in the underground. Asriel would have at that point matured into the cusp of an adult boss monster, both Toriel and Asgore would be nearing death’s door from their Soul power flowing into Asriel and either one of them could have offered their Soul to Chara at that point.
I mean it makes more sense to plan around having an adult human be the ambassador then a vulnerable child.
Also a human having a boss monster Soul is a completely unprecedented think apparently in the world of Undertale. We have no idea what kind of power boost or affect it has on humans.
But the point I’m making is that Chara could no longer see themselves living happily in the Underground anymore. They probably thought themselves as unsafe and at risk of hurting one of their adopted family again. So they removed themselves as a potential risk and gave their Soul to Asriel to enact on a plan that they thought could save the monsters.
(but you know…it didn’t.)