Very simple P-controller based autopilot derived from the one-liner:

let a = setInterval(() => {
// retrieve values from simulation
let roll = fixedRotationZ;
let pitch = fixedRotationX;
let yaw = fixedRotationY;
let rpy = new THREE.Vector3(roll, pitch, yaw);

let rollRate = -rateRotationZ/10.0;
let pitchRate = -rateRotationX/10.0;
let yawRate = -rateRotationY/10.0;

let rpyRate = new THREE.Vector3(rollRate, pitchRate, yawRate);

let pos = camera.position.clone();
let posRate = motionVector.clone();
let targetPos = issObject.position.clone();

let targetRpy = new THREE.Vector3(0,0,0);
let dRpy = targetRpy.clone().sub(rpy);

// P-controller controls target roll,pitch,yaw-rate (targetRpyRate) to reach target orientation
let targetRpyRate = new THREE.Vector3(
dRpy.x/1.0,
dRpy.y/1.0,
dRpy.z/1.0
);

let minRpyRate = new THREE.Vector3(-1.5, -1.5, -1.5);
let maxRpyRate = new THREE.Vector3(+1.5, +1.5, +1.5);
targetRpyRate.clamp(minRpyRate, maxRpyRate)

let dRpyRate = targetRpyRate.clone().sub(rpyRate);

// don't calculate with displayed x,y,z but the internal coordinates
// (z-axis is forward and backward)
let dPos = targetPos.clone().sub(pos);
let d = dPos.length();
// P-controller controls target motion (targetPosRate) to reach target position
let targetPosRate = new THREE.Vector3(
dPos.x / 100.0,
dPos.y / 100.0,
dPos.z / 100.0
);
let minPosRate = new THREE.Vector3(-0.1, -0.1, -0.01);
let maxPosRate = new THREE.Vector3(+0.1, +0.1, +0.01);

// thresholds for bang contRol
let dRpyRateControlThreshold = new THREE.Vector3(0.0, 0.0, 0.0);
let dPosRateControlThreshold = new THREE.Vector3(0.001, 0.001, 0.001);

// define some phases where we want to have different movement behaviour

if (dRpyRate.length() > 0.1)
{
// not correctly oriented, stop movement
targetPosRate.x = 0;
targetPosRate.y = 0;
targetPosRate.z = 0;
}
else if (Math.abs(dPos.z) < 20)
{
// we are very close, slow approach, larger P-control in lateral
// directions to correct remaining errors
minPosRate.z = -0.005;
maxPosRate.z = +0.005;
dPosRateControlThreshold.x = 0.01;
dPosRateControlThreshold.y = 0.01;
targetPosRate.x = dPos.x;
targetPosRate.y = dPos.y;
}
else if (Math.abs(dPos.z) < 50)
{
// we are getting, close, slow down
minPosRate.z = -0.01;
maxPosRate.z = +0.01;

minPosRate.x = -0.01;
minPosRate.y = -0.01;
maxPosRate.x = +0.01;
maxPosRate.y = +0.01;
}
else if (Math.abs(dPos.z) < 100)
{
// slow down
minPosRate.z = -0.1;
maxPosRate.z = +0.1;
}
else if ((Math.abs(dPos.x) < 0.05) && (Math.abs(dPos.y) < 0.05))
{
// lateral position correct, use maximum approaching speed
minPosRate.z = -0.2;
maxPosRate.z = +0.2;
}
targetPosRate.clamp(minPosRate, maxPosRate);
let dPosRate = targetPosRate.clone().sub(posRate);

// bang control to reach target roll pitch and yaw rate
if (dRpyRate.x < -dRpyRateControlThreshold.x)
rollRight();
else if (dRpyRate.x > +dRpyRateControlThreshold.x)
rollLeft();
if (dRpyRate.y < -dRpyRateControlThreshold.y)
pitchDown();
else if (dRpyRate.y > +dRpyRateControlThreshold.y)
pitchUp();
if (dRpyRate.z < -dRpyRateControlThreshold.z)
yawRight();
else if (dRpyRate.z > +dRpyRateControlThreshold.z)
yawLeft();

// bang control to reach target motion
if (dPosRate.x < -dPosRateControlThreshold.x)
translateLeft();
else if (dPosRate.x > +dPosRateControlThreshold.x)
translateRight();
if (dPosRate.y < -dPosRateControlThreshold.y)
translateDown();
else if (dPosRate.y > +dPosRateControlThreshold.y)
translateUp();
if (dPosRate.z < -dPosRateControlThreshold.z)
translateForward();
else if (dPosRate.z > +dPosRateControlThreshold.z)
translateBackward();

},100);