Don’t worry, staying where you are doesn't put you at risk. Since TD Ameritrade has fully merged into Charles Schwab, you’re backed by a 100% security guarantee against unauthorized activity.
Vanguard is the gold standard for long-term safety; however, its interface can feel a bit clunky for daily use.
You should stay with Schwab/TD for the awesome thinkorswim tools, but remember to set up two-factor authentication (2FA) — it’s your best line of defense, no matter what broker you choose.