How do I transfer data from old external hard drives to iCloud?

Your choices really are:

• Enable iCloud Drive on your computer and have the files on your Mac (you could optimise them to save space).
• Disable iCloud and load the files to iCloud through your browser, to avoid having the files on your computer.
• Use another cloud service for the files that does sync to your computer.
• Use another Apple ID, add it to a family share and load files through the browser. This would let you put the files into iCloud under another ID, but let you continue to use iCloud under your own ID without the files being on your computer.