Split 3D polyline at point using QGIS

I want to split a QgsPolyline geometry into two separate lines at a given QgsPoint (by projection onto the line), adding the point to the line as end point of the incoming line and start point of the outgoing one.

I got the following code that works for 2D geometries (creating a QgsPointXY point and a QgsPolylineXY poly from the input QgsFeature):

def breaklineatpoint(pointfeat, polyfeat):    point = pointfeat.geometry().asPoint()    poly = polyfeat.geometry().asPolyline()    geo = QgsGeometry().fromPolylineXY(poly)    (sqdist, projpoint, aftervertex, leftOf) = geo.closestSegmentWithContext(point)    newline1 = poly[:aftervertex]  # from start to break point    newline1.append(projpoint)  # append point    newline2 = poly[aftervertex:]  # from break point to end    newline2.insert(0, projpoint)  # insert at position 0    return newline1, newline2

However, it drops the feature's Z and M values. How do I translate this function so that it can handle 3D geometries (QgsPolyline instead of QgsPolylineXY and QgsPoint instead of QgsPointXY)?

I already fail creating a QgsPolyline from a QgsGeometry. Using a QgsLineString by QgsGeometry().constGet() is no alternative as I then can't apply the Python list cutting and slicing functions.