I have just finished SharePoint Online manual migration. And this post is all about migration Term Store and the managed metadata columns.
So, you have got a term set on-prem.
How do you migrate the term set to SharePoint Online?
In case of migration to SharePoint Online manually, the only option you left with - is Import CSV file of term set to the destination - SPOnline.
The provided PowerShell script exports the term site into CSV file on-prem in order to import it in SharePoint Online.
The pitfall of importing CSV file , you can't specify child-parent relationship between term sets.
Another issue that I have discovered that there is no way to copy over Keywords, since it's a system term set. So, in case you need them from on-prem either you should re-type them manually or (much better option) a buy a 3rd party tool for SharePoint migration.
And finally, the BIG ISSUE of manual migration of managed metadata:
Imported terms will get a different GUID than terms on-prem. That means, every list that has managed metadata after migration should be updated manually with valid imported term.