You'll need to run the math to see how much of an impact the ER difference would be and compare that to the tax savings (don't forget to consider the tax savings as additional investments).

Can you share which state you're in, your state tax rate, and amount you plan to contribute? It's a math problem of whether the drag from bad investment options/high fees outweighs the tax savings from contributing to the eligible state plan. In most cases, the tax benefits still win.

You should also be aware, you can't just open a "fidelity" 529. Fidelity manages a few state plans (Arizona, Mass, Connecticut, Delaware, and New Hampshire), and for non-residents who still want a fidelity-managed plan, they route them to the NH.

Though even in that plan, you don't get access to the full suite of Fidelity funds. They have a few options for managed portfolios that you can choose.

Same goes for Vanguard plans. They manage New York, Colorado, and Nevada's plans, and route non-residents to their Nevada plan.

Our state was Vanguard, and Fidelity is no better or worse than Vanguard. If you despise the Vanguard then don't worry about the state tax deduction and go with Fidelity.

Some states have BAD ones though. I rate Vanguard and Fidelity run ones as good ones.