See above results with comments.

Oh, sorry but I made the same mistake as you!
This only works for property which have been declared with Q_PROPERTY, and therefore added in the QMetaObject create by the MOC!!

What you are looking for is QObject::dynamicPropertyNames() (cf https://doc.qt.io/archives/qt-4.8/qobject.html#dynamicPropertyNames)

See above results with comments.

Oh, sorry but I made the same mistake as you!
This only works for property which have been declared with Q_PROPERTY, and therefore added in the QMetaObject create by the MOC!!

What you are looking for is QObject::dynamicPropertyNames() (cf https://doc.qt.io/archives/qt-4.8/qobject.html#dynamicPropertyNames)

clsJSON::clsJSON(QObject* pParent) : QObject(pParent) {
}
void clsJSON::insert(const QString& crstrMember, const QVariant& crvarData) {
    if ( crstrMember.isEmpty() == true || crvarData.isValid() != true ) {
        return;
    }
    setProperty(crstrMember.toLatin1().data(), crvarData);
}
QString clsJSON::toString() {
    const QMetaObject* cpobjMO(metaObject());
    int intOffset = cpobjMO->propertyOffset()  // returns 1
       ,intCount = cpobjMO->propertyCount();    // returns 1
    const QString cstrTest("Another");
    const QVariant cvarData("test");
    insert(cstrTest, cvarData);
    QVariant varP1(property("HELLO"));       // Works and varP1 contains "WORLD"
    QVariant varP2(property("Another"));    // Also works varP2 contains "test"
    intOffset = cpobjMO->propertyOffset()  // still returns 1
    intCount = cpobjMO->propertyCount();    // still returns 1
    for( int i=cpobjMO->propertyOffset(); i<cpobjMO->propertyCount(); ++i ) {
        ...  
    }
    return ...;
}

See above results with comments.

[Edit] See additional to lines where I get the property data inserted and verify it is correct, yet the propertyOffset and propertyCount are both still 1.

clsJSON objDemo;
objDemo.insert("HELLO", "WORLD");
QString strJSON(objDemo.toString());
qDebug() << strJSON;

The class itself:

clsJSON::clsJSON(QObject* pParent) : QObject(pParent) {
}
void clsJSON::insert(const QString& crstrMember, const QVariant& crvarData) {
    if ( crstrMember.isEmpty() == true || crvarData.isValid() != true ) {
        return;
    }
    setProperty(crstrMember.toLatin1().data(), crvarData);
}
QString clsJSON::toString() const {
    const QMetaObject* cpobjMO(metaObject());
    for( int i=cpobjMO->propertyOffset(); i<cpobjMO->propertyCount(); ++i ) {
        ...  
    }
    return ...;
}

toString is not compete yet, but the loop isn't getting iterated at all, I've checked and as posted propertyOffset and propertyCount both equal 1.

Where crstrMember contains "Hello" and crvarData contains "World". When I get to the loop, both propertyOffset and propertyCount return 1, so it will not iterate at all.

Are you sure you are testing on the right object instance?
Note: propertyOffset = 1, which is normal, property 0 is the name() property of the QObject (cf. https://doc.qt.io/archives/qt-4.8/qmetaobject.html#propertyOffset).

setProperty("Hello", "world");
const QMetaObject* cpobjMO(metaObject());
for( int i=cpobjMO->propertyOffset(); i<cpobjMO->propertyCount(); ++i )
    qDebug() << "Found" << i << "as" << QString::fromLatin1(cpobjMO->property(i).name());