The most efficient way to share documents?

Hi Alex!

That was a difficult question and the answer is "It depends" :-)

There is no silver bullet for how to implement SharePoint. Depending on what your needs are there are different answers. When i use SharePoint now is often for

Intranets and then i set up Communication Sites for news publishing and common internal document sharing.

When it comes to "Project sites" i prefer to use Microsoft Teams. When You create a team, there will also be created an Office 365 Group and a SharePoint Site. The team members have access to everything in the team (and its SharePoint site) so if You, like You mention, have very specific access restrictions it can be difficult to handle that with/in Teams.

If your access restrictions are very specific You can handle the access in many levels using SharePoint (Site Collection, Site, Library, folders, items). My advice is to handle the access on an as high level as possible, otherwise it gets messy very soon.

Using SharePoint for Project Sites and Site- and Document Sharing with both Internal and External users i recommend that You split them between Internal and External Site Collections ( this is the "old way") so that You have "Internal Projects" and "External Projects", then You can configure the internal site collections so that their subsites and their content are not possible to share with external users by accident. Have You considered using https://support.office.com/en-us/article/learn-about-office-365-groups-b565caa1-5c40-40ef-9915-60fdb2d97fa2?

Regardless of what You choose to use it takes a great deal of planning in advance. If you go for classic SharePoint i recommend that you start by reading "https://support.office.com/en-us/article/sharepoint-online-planning-guide-for-office-365-for-business-d5089cdf-3fd2-4230-acbd-20ecda2f9bb8" but based on the needs i think You have the best solution for You is to use Microsoft Teams/Office 365 Groups.

Hope that this helped You a little instead of making You confused on a higher level.

Best regards and good luck, Magnus