If you have lot of navigation links, ellipses (...) will be shown as per the screen size (due to responsiveness). I don't think you can avoid this on SharePoint online site.
So, try to use the combination of menu & submenu for navigation in SharePoint online.
If you want to show multiple submenu/subsite links, you can use Megamenu navigation style which will look like below (on hover of main menu link, no additional click needed):
You can change navigation style from Change the look settings, Change the look of your SharePoint site.
Reference: Customize the navigation on your SharePoint site