Unable to sync library or open in Explorer View because there are too many files

You could ask your admin if they have daily time window set up for when large queries are allowed, but I believe this is only an option if you have the on-premises version of SharePoint. You can read more about this in https://support.office.com/en-us/article/Manage-large-lists-and-libraries-in-SharePoint-2013-and-SharePoint-2016-b8588dae-9387-48c2-9248-c24122f07c59and then choosing "Using the daily time window" under the heading Ways to manage lists and libraries with many items.