I don’t think your word count is a problem. I’ve seen longer demos posted on this forum (at least the ones on Dashingdon before the full version was released on HG/CoG). Public betas are (cmiiw) mainly for finding bugs and other major issues, i.e. making sure the game is playable and not rife with issues upon release. You could do a private beta if revealing the entire story is a concern.
Might want to check out this thread as well:
https://forum.choiceofgames.com/t/average-demo-word-count