FXIAX is different from VTSAX. FX is SP500 only and VTSAX is total US market. The problem, as you know, is with the damn fees. Can you look up and post what the fees are for the various funds?
Do you have "brokeragelink" available with your 401k? That's a thing that fidelity offers that lets you control your investments into stocks, mutual funds, ETFs, etc. It's like a brokerage account but with your 401k. Ask your 401k/HR people or check your plan literature if you have it.
If you have that, then you can invest in the fidelity equivalent of VTSAX with microscopic fees.