Site collection size limit

As Juan responded the 200 GB is not a hard limit. It depends on the IOPS of the underlying SQL server but also on the backup/restore capabilities of the SQL server.

There are organisations out there that have 100's of TB stored in SharePoint. So it can be done.

I would indeed split the current data volume into smaller parts, i.e. use multiple site collections to keep the content database sizes at an acceptable level (say 200-300 GB unless your SQL administrators tell you they have the IOPS to use larger sizes.

Metadata
You mentioned the files contain metadata? are we talking Office files like docx or other types (pdf, msg, eml, ..)?
Capturing these metadata values into SharePoint columns would improve the search experience for your users.

Migration
You can use PowerShell or migration tools. There are plenty of them. see e.g. https://directory.collab365.community/office365-sharepoint/office-365-migration-software/

I would not consider moving such a large data volume using explorer view. Throughput will be insufficient and there is no logging.

SQL Server
You need to involve the SQL Administrators. They need to be aware of the coming increase in database files. and also cater for large transaction logs during migrations.

Paul | SLIM Applications (http://www.slimapplications.com/