Any potential concern of large SharePoint document libraries (>5,000)?

Agree with most points. Definitely try to break into multiple libraries due to sharing permissions, unless you turn Sharing off. We haven't hit the issue yet, but trying to redo any custom permissions once the chain gets broke and you have over 10k? Might be 100k can't remember, you can't set permissions at that point at that high up the hierarchy.

The one biggest thing you want to do first off is disable sync. Sync is the devil and the old tech used for OneDrive which would sync entire libraries and when stopping sync and syncing other folders will duplicate files locally on users machines and they will start editing the non synced files locally from "recent files" in office apps etc. It's a nightmare. So turn Sync off if you take any advice from this thread lol.

This doesn't mean they can't sync thou. They can still utilize the "Link to OneDrive" feature which is way better tech. When you remove the link/shortcut etc. it will purge all the synced files from the client that are in the cloud and cleanup etc. you can easily selectively sync folders AND it syncs to the cloud onedrive creating shortcuts to those locations so if you have mulitple machines or replace machines the synced locations to SharePoint shortcuts/links will migrate simply by setting up OneDrive.