Can I use SalesforceDX to load data into sandboxes?

You also can take a look at the SFDX plugin called SFDX-Data-Move-Utility. I took part in development of this plugin, which is hosted in the forcedotcom GitHub account under the BSD 3-Clause license type. This tool can be very handy for loading the data from another org or from .csv files.

The main advantages of this tool:

• It can transfer or load multiple sobjects in a single task without your interaction.
• It can process multiple SObject types preserving relationships between them. Even circular references are supported. This will save time when you have to transfer complex data model with many objects and a lot of lookups between objects.
• There is no size limitation on your data sets. No workaround needed to transfer big data as when you working with tree:import.
• With this tool you can also make direct org-to-org data transfer without intermediate .csv files.
• There is native support for transferring person accounts/contacts and record type.
• The tool provides option to customize field mapping between the Source and the Target.
• There is built in support of data anonymization if you want to mask sensitive production data while transferring.
• The tool is fully configurable with simple JSON file.