I would also keep an eye on the PC resources and make sure they have sufficient RAM for the syncing and multitasking. If you're syncing to the default location, consider making an exception for any antivirus software. We don't share devices, so this works well for us and provides a consistent file path across PCs for shared files. We have a folder on the C: drive 'OD' and everything syncs to that location rather than the default %appdata% folder. We also have a non-standard setup for OneDrive app. I don't think there is anything we changed on the sites themselves. With decent internet connections its not a problem to open online-only documents. You can divide your current directories into multiple libraries and only sync what is necessary. We're well past 300k items in a few libraries let alone all of them.

It would appear that this is not a solution that will work for our company. If you use the previous OneDrive for Business sync client (Groove.exe), the sync limit per library is 20,000 items (including 5,000 items per team site). Additionally, the same performance issues can occur if you have 300,000 items or more across all libraries you are syncing, even if you are not syncing all items in those libraries. Although SharePoint Online can store 30 million documents per library, for optimum performance we recommend syncing no more than 300,000 files across all document libraries. For optimum performance, we recommend storing no more than 300,000 files in a single OneDrive or team site library.

G suite drive sync limit