Solutions for file sharing - local office plus cloud

Question

We have a main office of 8 computers, 3 Windows 10 PCs and 5 Mac desktops and laptops. We have a MacOS server as well.

We have about 200 GB of files on local network shares from the Mac server. Increasingly, this is a paint point. Access to the shares on the LAN is fine, of course. But we now have remote staff who reside all across the country, and even some of our local staff work occasionally from home.

I'm looking for a solution for providing the best and simplest access to all files.

What I have tried:

Network Shares

Work OK for computers on the LAN. Using WebDAV, access is exactly the same on campus and off-campus. However, WebDAV is noticeably slower than SMB2 on campus, and SMB over VPN is intolerably slow. Access on iOS devices is not great.

SharePoint as the central repository, sync files to server using OneDrive for Business, share OneDrive folder across LAN

We have an Office365 subscription that comes with lots of SharePoint storage. But, I ran into all manner of permissions issues, and not all files were available off campus on the Macs (only MS Office files were accessible) unless the user synced folders (in which case a single user editing a file would trigger everybody in the office downloading a new copy—and required all folders to be synced to user machines which isn't helpful and raises issues about backups, etc. etc.).

What I'm really looking for

How to streamline/improve access for phone/tablet users and local PC/Mac users on the LAN; or how to browse a Sharepoint site from Mac finder; or another solution altogether that will provide a smooth experience for local and remote users (most not technically savvy, who are accustomed to browsing network shares like they browse their local file hierarchy).