SharePoint online as File Server

Interesting discussion, which hopefully I am understanding correctly.  Firstly I'd suggest taking a step back, I'd say don't try to migrate 1.5TB of data (if that's what you are saying), it's just my personal opinion rather than a rule.  1.5TB is a huge amount to bring across to SharePoint Online and if your migrating anything like that, a migration tool might be in order.  

Dumping stuff unorganised into SP (I am not saying you're doing that) without looking at governance and Information Architecture isn't going to end well. This goes into some of the finer points - https://support.office.com/en-US/article/Overview-best-practices-for-managing-how-people-use-your-team-site-95e83c3d-e1b0-4aae-9d08-e94dcaa4942e.

Using SharePoint as a file server is not without its issues anyway, for example, there is a 5000 https://support.office.com/en-us/article/manage-large-lists-and-libraries-in-office-365-b4038448-ec0e-49b7-b853-679d3d8fb784 unless that's changed recently. You'd want to cleanse the data anyway if that hasn't happened already, what can be archived, what doesn't need to be kept and certain things that just aren't suitable for SP. This post has a nice set of steps and list of migration tools - https://blogs.technet.microsoft.com/ptsblog/2013/11/04/migrating-file-shares-to-sharepoint-online/.

I am not sure I understand the role of the NAS, though you have explained it, I'd personally try to eliminate its unless there is no other way.  Syncing Sharepoint files with the OneDrive sync client is https://support.office.com/en-gb/article/Sync-SharePoint-files-with-the-new-OneDrive-sync-client-6de9ede8-5b6e-4503-80b2-6190f3354a88, there is also the upcoming https://blogs.office.com/2017/05/11/introducing-onedrive-files-on-demand-and-additional-features-making-it-easier-to-access-and-share-files/ features (Win 10 only), if any of those may help. The modern http://www.windowscentral.com/new-onedrive-app-windows-10-pc is a quite nice alternative to syncing.