PyQgis 3 - Add New Layer to Spatialite Database

Question

For the most part, working with QGIS layers has been a pleasure, but I've run into some difficulties recently when trying to create a new vector layer in the SQLite database I've been using.

I've created a new database with the QgsVectorFileWriter like this:

writer = QgsVectorFileWriter(
    "/path/to/db/testdb.sqlite",
    "utf-8",
    myFields,
    QgsWkbTypes.PointZ,
    QgsCoordinateReferenceSystem(
        4326, QgsCoordinateReferenceSystem.EpsgCrsId),
    "SQLite",
    ["SPATIALITE=YES"])

And noticed that QGIS creates a default new table called "testdb" (named after the database) that works fine to load and save as to a layer.

I want to create a new layer with a new table now in the same database, so I went over to the docs at https://qgis.org/api/classQgsVectorFileWriter.html.

---

Using QgsVectorFileWriter()

I saw there was an ActionOnExistingFile parameter in the constructor to create a new layer & table, only to be disappointed when I noticed this parameter had been mysteriously dropped in the python implementation of the QgsVectorFileWriter, according to the pyQgis docs.
Feeling hopeful, I threw in an action=QgsVectorFileWriter.CreateOrOverwriteLayer keyword, and even tried typing out all 11 arguments to avoid the keyword argument, but no luck.

# Doesn't work
writer = QgsVectorFileWriter(...
    action=QgsVectorFileWriter.CreateOrOverwriteLayer)

Using QgsVectorFileWriter.writeAsVectorFormat()

I did see that the python writeAsVectorFormat method had an option to specify the new table action, so I went ahead and wrote up something like this, using the SaveVectorOptions class:

options = QgsVectorFileWriter.SaveVectorOptions()
options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer
options.layerName = "testlayer"
writer = QgsVectorFileWriter.writeAsVectorFormat(
    layer,
    "/path/to/db/testdb.sqlite",
    options)

But... The layer argument assumes I already have a valid layer from a valid data source. I need to create the source first before I can create the layer though, right?

Using a memory layer...?

If one can't create the data source first, I thought maybe loading a layer with a temporary "memory" provider and then writing it to the database would be an option. I'm not sure how I would specify the other parameters that I need for the vector layer (such as the fields and the CRS) using this:

layer = QgsVectorLayer("PointZ", "testlayer", "memory")

and then saving it to the database. I'm not even sure whether this method would work, but it seems a little hacky to me in any case. But I could be wrong...