For the really paranoid, to the extent that your data can be represented as a text file, you can print it on paper from your internet connected machine and OCR

For the really paranoid, to the extent that your data can be represented as a text file, you can print it on paper from your internet connected machine and OCR it into your air gapped machine, and vice versa. In this case, you only have to worry about your printer or scanner having a backdoor. If you are very confident in your OCR accuracy, you can encrypt it prior to printing and decrypt it after scanning. Just remember to burn the paper afterwards.