I sometimes respond weekends, but no guarantees ;-) I am happy it is appreciated.
I’m not certain of the situation you describe “I have a backup of some files on that external hard drive; backup occurs once a month. In the above scenario, that backup would be encrypted. Therefore, I’ll have to decrypt, do the backup, then encrypt again; am I right?”
If the files are encrypted on the external hard drive with AxCrypt (or similar file encryption software), then they stay encrypted regardless of how you copy or move the files. So, no, in that situation you backup the files like any other files. They are encrypted on the external disc, and are identically encrypted on the backup copy. If, however, you encrypt them with EFS or similar ‘transparent’ encryption, they will normally be decrypted transparently on read, and thus be written to the backup unencrypted, unless that destination again is configured for encryption.
With AxCrypt currently encrypting 300GB should take about as long as copying the data twice, more or less. It depends on the size of the files, the speed of the storage media and the speed of the processor.