it’s still only installing as a few kB for me on tinfoil when the files like 2gb ♂️
The old placeholder NSP file for the first DLC is only a few KB, but that is not the file you need for the DLC. If your Tinfoil shop doesn't have the ~2 GB version of the DLC package, then you need to download it from another source and install it manually. I recommend using something like Awoo-installer when installing NSP files manually, since Tinfoil doesn't play nicely with a lot of NSP files.
Since it's only about 2 GB, you could always download it onto your phone/table (whatever device you have that isn't a computer), send it to your Switch's SD card, and install it that way.