Depends on where you have your HSA. No one is forcing you to leave your HSA $$ at your employer’s HSA provider, and in fact you can maintain multiple HSA accounts if you like. The main catch is that you’ll want to keep the tax advantages for contributions into an HSA so initially you should still use the employer’s HSA provider for that (to get that payroll deduction + employer match if they offer it).
e.g. my employer’s HSA provider’s investment options aren’t great so I only leave a small amount there & transfer the rest of my collected HSA $ into a Fidelity HSA account. Fidelity does not charge account maintenance fees AFAIK.
EDIT: For OP if they no longer want to use their employer’s HSA, & they’ve stopped contributing to it, they may as well transfer the entire balance over to Fidelity or some other HSA provider with $0 fees & invest there. It’ll be similar to maintaining an IRA account.