It is tempting to not disclose a new job, but per the severance agreement (which I am still trying to clarify with HR), you still get the severance if you get a new role outside Schwab, but you don’t get the bonus of you get the role before the end of the year.
This may not be a popular thing to say, but it is best to be upfront and disclose of you get another role. At the very least, you will not have the risk of legal action taken against you. Forgoing the bonus is worth keeping your next role, in my opinion.