Grid-forming inverters-based autonomous microgrids present new operational challenges as the stabilizing rotational inertia of synchronous machines is absent. We propose in the paper a control architecture for frequency and voltage control with good scalability properties. At slower timescales, it allows to incorporate a distributed secondary control policy for which we provide an analytical stability result with line conductances taken into account. At faster timescales, it satisfies a passivity property for a wide range of parameters. The distinctive feature of the voltage control scheme is that it has a double loop structure that uses the DC voltage in the feedback control policy to improve performance. The frequency control policy employs the inverter output current and angle to provide an improved angle droop policy. The performance of the control policy is illustrated via advanced simulations.