What is the best method to import content from existing mailboxes to new test mailboxes?

They say never answer a question with another question so here you go 🙂

you could use EAC to Export and Import a PST from source user mailbox to destination user but PowerShell would be your best bet. Either way, you will need the "Mailbox Import Export" role permission to be assigned to your account for this to work (don't forget to close your sessions and start a new browser session when done).

Create mailbox import requests
Use the EAC to create a mailbox import request
In the EAC, go to Recipients > Mailboxes > click More options More Options icon., and select Import PST.

In the EAC, select Recipients, Mailboxes, and then select More Options.

The Import from a .pst wizard opens. On the first page, enter the UNC path and filename of the source .pst file.

In the Import from a .pst file wizard in the EAC, specify the source .pst file.

When you're finished, click Next.

On the next page, select the target mailbox, and then select one of these options:

Import to this mailbox

Import to this mailbox's archive

In the Import from a .pst file wizard in the EAC, select the target mailbox (primary or archive).

When you're finished, click Next.

On the last page, configure one of these settings:

Leave the Send email to the mailbox below when the .pst file has been exported check box selected. Click Browse to add or remove notification recipients.

Clear the Send email to the mailbox below when the .pst file has been exported check box.

In the Import from a .pst file wizard in the EAC, select whether to receive notification messages.

When you're finished, click Finish.

Steps are in this KB - https://learn.microsoft.com/en-us/exchange/recipients/mailbox-import-and-export/import-procedures?view=exchserver-2019