An example of how to convert Euler to Quaternions and use slerp for rotation.

rotation-example-slerp.js

// Rotations
const step = 'q1';
const cameraRotations = {
  q1: {
    x: 0.75,
    y: 0,
    z: 0,
  },
  q2A: {
    x: -0,
    y: 0,
    z: -0,
  },
  q2B: {
    x: -0,
    y: 0,
    z: -0,
  },
  q3: {
    x: -0,
    y: 0,
    z: -0,
  },
  q4: {
    x: -0,
    y: 0,
    z: -0,
  },
};

const vecR = new THREE.Quaternion();
const tEuler = new THREE.Euler();
const tRotVals = Object.values(cameraRotations[step]);
// Set latest Euler rotation values
tEuler.set(...tRotVals, 'XYZ');
// Convert the Euler rotation to a Quaternion rotation
vecR.setFromEuler(tEuler);

useFrame(({ clock, camera }, delta) => {
  if (!DEBUG_MODE && rotations[step]) {
    cam.current.quaternion.slerp(vecR, 0.075);
    ...
    cam.current.updateProjectionMatrix();
    cam.current.updateMatrix();
  }
});