External sharing good practice

I seldom recommend using the subsite approach, you have much stronger controls creating separate site collections for each company/partner that you want to share with. This is very easy to explain to most people.

If you have a project with multiple subcontractors, creating separate document libraries for each one of them is a very clean and easy way to provide the necessary security controls. Create a SP Security Group for each firm, and then align it with a document library, have some other libraries for content that needs to be shared with everyone. 

Many of my clients block external sharing from OneDrives and only allow it from selected SPO Site collections.