Retrieve all metadata from an existing sandbox (for Salesforce DX conversion)

This is what I would do:

1. Create a project in Force IDE or Mavensmate connecting to your sandbox and select all the metadata components and retrieve metadata. Trick here is that Force IDE and Mavensmate also creates package.xml

2. Now use the package.xml created in above step and use the below command as suggested in this answer

   sfdx force:mdapi:retrieve --retrievetargetdir force-app -k package.xml