There used to be the Western Digital My Passport Wirelesse. External hard drives with SD readers. They don't seem to be available any more. I stick to my tip: take enough memory cards with you and that's it.

With this device you can transfer the pictures and videos from the camera's memory card to the computer. I do this transfer once a day. Immediately after the transfer, I sort out the bad photos. I like to work with Capture One or Lightroom. Now the photos are saved on the computer and you can delete the memory card in the camera. Depending on the amount of photos, you can also choose a different workflow.

It's just a card reader. It has no built-in memory. i would just take enough big memory cards with me (256 GB).
However, external disks with built-in card readers are also available.