Consolidating into one site collection

That's far from the recommended approach these days :P. I was the total opposite of you. I went from 1 site collection to many site collections. Especially with the advent of Teams. You may end up eliminating those units in place of Team connected site collections. I don't know your big picture but if you do plan on using Teams I would focus your migration around Teams. vs. moving SharePoint then integrating Teams after the fact cause it will make it more difficult to move things around.

Every entity you create in Office 365 these days is a site collection. Everything you create is tied to an Office 365 group, which also creates a site collection with it. So in my case, I now have a Teams for every department, a team for every sub team inside the department. Which creates site collections for both. Then org facing sites that are communication sites for org consumption, these are their own site collections.

Hub sites help connect all these flat site collections together for providing consistent branding and news/content rollup to the hub site. Think Parent site with sub sites, but they aren't connected in any way share or form other than using the parents nav, and theme across all sub sites.

So not sure about that article, seems old thinking, the new way is many site collections, no sub sites, since that's how Microsoft likes to scale these days with SPO.

The limits are fairly large per Site collection these days,, looks to be up to 25TB according to this: https://technet.microsoft.com/en-us/library/mt842345.aspx but the beauty of 365 is you don't have to think about content databases and those things anymore as an admin :).