Sure. This isn't a hot take. The legislature can order the executive to take specific actions. It does so all the time. The reason every society in history separates or delineates its executive functions is because many tasks of government require faster action than a deliberative body can continuously commit to.
Congress (big and slow) granting agencies (small and faster) limited rule-making powers increases the agencies' power. The executive granting the Congress limited executive powers (e.g. prosecuting contempt of Congress) doesn't confer an equal power transfer. The executive branch of any system is built to amplify and focus power; the legislature, to distribute it. That executive power, if vested in the Congress, would dissolve across the body.
An FTC under the Congress would have the same authority as any Congressional subcommittee: infinite in theory, but relatively toothless in practice.