Is it correct to say that constant speed = 0 acceleration = no resultant force?

Constant speed does not mean zero But zero acceleration does mean zero resultant force.

Speed is a scalar. Instantaneous speed is just the magnitude of the velocity (which is a vector by the way). So motion could have a constant speed but a varying velocity. Uniform is one example of this.

Now acceleration is the rate of change of velocity not the rate of change of the speed. So we can have constant speed, changing velocity and thus NON ZERO acceleration.

In summary,, acceleration is needed not only to change speed but to change direction as well, so whenever direction changes you need an acceleration and also therefore a resulting force.