You can add an OnUpdate to the tween like this:
Tweener tweener = target.DOMove(followTarget.position, speed).SetSpeedBased(true);
tweener.OnUpdate(delegate () {
// if the tween isn't close enough to the target, set the end position to the target again
if(Vector3.Distance(target.position, followTarget.position) > completionRadius) {
tweener.ChangeEndValue(followTarget.position, true);
}
});