if you have investments at fidelity you can just use the brokerage account as cash management. the brokerage account can access funds using a debit card and ACH

if you have investments at fidelity you can just use the brokerage account as cash management. the brokerage account can access funds using a debit card and ACH just fine; I do still recommend opening a CMA for ATM fee reimbursements though. none of these funds availability problems can happen if you have margin enabled; note that you won't pay any margin interest either as funds on hold are tradable immediately.