As you found, waitForObjectItem() pre-dates and does not support QML/QtQuick. (Unfortunately the manual does not say that there.)
It looks like you are manually writing your automation script. This is something that I advise to do only after knowing the basics of Squish, for which I recommend to go through the tutorial of Squish for Qt first, at minimum.
Furthermore, if you don't know how to solve something, I recommend to start up the Squish IDE and to record the desired interaction. Afterwards, in an ideal case, you have a recorded script snippet that replays properly. And this script snippet and the object names used in it you can then study to learn how to do what you need in your manually written script code.
As for accessing a QML/QtQuick ComboBox, this is what Squish is recording for me in case of the Qt example "quickcontrols2/gallery":
def main()
startApplication(os.path.expanduser("~/qt/5.15.2/gcc_64/bin/gallery"))
# Open menu:
mouseClick(waitForObject(names.image_IconImage_2), 11, 15, Qt.LeftButton)
# Select menu entry "ComboBox":
mouseClick(waitForObject(names.comboBox_ItemDelegate), 48, 25, Qt.LeftButton)
# Expand ComboBox control:
mouseClick(waitForObject(names.page_ComboBox), 104, 13, Qt.LeftButton)
# Click on one of the items:
mouseClick(waitForObject(names.o_Rectangle), 55, 24, Qt.LeftButton)
# Expand ComboBox control:
mouseClick(waitForObject(names.page_ComboBox), 110, 13, Qt.LeftButton)
# Click on one of the items:
mouseClick(waitForObject(names.o_Rectangle), 63, 22, Qt.LeftButton)
As you can see, this is not "high level" ("select entry X in this ComboBox"), instead, it records interactions with objects from the object tree.
There is nothing that will stop you from writing convenience functions like selectComboBoxEntry(obj, itemName), but you will have to do this yourself. And for this you need to understand the basics of Squish, the object tree, object names and object identification, and so on. (All of these can be found in the manual.)