On our SharePoint config we've got a subsite for each client that we pick up. When creating we select to display the site on the Quick Launch of the parent, which works exactly as expected. Except ... that the sites aren't in alphabetical order. Is there any way to automatically sort the list in SharePoint online? Everything I'm finding when I Google is about SharePoint on-prem.
Please have a look here hopes it helps you out - https://support.office.com/en-us/article/Configure-the-Quick-Launch-for-site-navigation-d3288963-4a91-4470-9192-610952daecdf . As per the requirement i think this should resolve your problem which you have asked.