Do I need a trusted setup?

Only if your proof system requires a structured reference string. Hash-based systems — STARKs and other FRI-based constructions, Bulletproofs, and IPA-based schemes such as Halo2's original instantiation — need none. Groth16 needs a circuit-specific setup, so a new ceremony is required whenever the circuit changes. PLONK, Marlin and KZG-based Halo2 need a universal, updatable setup that can be reused across circuits, and in practice teams reuse an existing public transcript rather than running their own. See Section 04.