I run Office 365 for small business and am struggling to find any useful real word advice/guides to help in planning and setting up the back end side of things to match in with the way I work. I have searched endlessly but always seem to end up back at the Microsoft sites with the generic high level stuff. I know that I can pay somebody to do this but I cant afford to have an "IT Guy" and I need to be able to manage it myself. Everything is set-up and running but it still a bit disjointed. The areas I am looking for information on are:

Central Storage Area - I have SharePoint but running remotely is a real pain with no local cache. I understand that Onedrive is supposed to cover that but I lose all of the functionality of SharePoint. Ultimately I would like a permanently mapped or local cache the files that can be worked or at least particular folders. I am ready to be told that this isn't possible without a local install of sharepoint but would feel quite mis-sold if this was the case.

External User Access - Being able to provide access to third parties so that they can access, upload and retrieve files from my sharepoint area.

User Management - Best approach for new user accounts and management. I generally have contract staff work for me where i need them to have an account to access and co-ordinate but not permanent and i would need to be able to ensure security of data when they use it and track what they are doing.

Archiving and Data Security

Onedrive For Business -
I run about 5 ODFB locations on different devices and seem to endlessly have sync issues. I have gone through the whole filename, file limit, illegal character phase and comply with everthing, but i am resetting my onedrive almost every couple of days on one device or another which cant be right.

Shared In-Box management for emails and calendar etc.

Separate Mail boxes for different projects or shared folders
- I'm thinking like the old IMAP folders, but with full outlook functionality.

Anybody got any good sources of information? or have a good set-up that they wouldn't mind sharing some tips? I also would prefer to have no third party add-ons or work arounds if possible, rather keep it all vanilla.