Dear respected Joyce,
Good day!!
I understand your concern but since you concern is related to SharePoint API, I suggest you post your concern in the related community: Questions - Microsoft Q&A with SharePoint Developmenttag to get the detailed help from the experts there.

Apologies for redirecting you to a different community as the community moderators in the category mainly focus on the out-of-the-box features in SharePoint Online platform and have limited knowledge on the customization or SharePoint API endpoint, so to get the fast and better assistance for your code of getting quick launch property, we have redirected you to the correct community.
In addition, I will keep this thread open so that SharePoint MVPs in this community may share their ideas and other possible troubleshooting suggestions on your scenario.
Thanks for your time and effort on checking my reply above. Appreciate your patience and understanding. Have a great day!!
Best Regards,
Mia