Content DB to large

Good day, I'm hoping someone can help me with my issue.  My client has on prem SharePoint 2016 and they are using one collection site as an intranet with a subsite for document and records management (EDRMS).

One particular subsite contains client information, a folder with the clients' name and account and then subfolders with all the scanned documents related to the client. This subsite, in the EDRMS is growing much larger and quicker than what they anticipated, in such a way that their Content DB is now actually way larger than what MS advices.

Since you can not have 2 content DB's for one site collection, I suggested that we create another site collection on their SharePoint with it's own content DB, keeping the active clients in the current EDRMS subsite and moving the inactive clients (which is much larger) to the new site collection.  Then we move that big subsite in the current EDRMS to a new EDRMS on the newly created site collection.

My question is:

1. Is this the best solution or is there other options we can explore

2. How do we move the content (each client has a folder with sub folders) from the current site collection to the new site collection?  (Unfortunately, it can not be done via a Powershell script, since the client needs to identify each folder that needs to be moved and then prefer to move it manually)