I have a 100's of GB of documents storage on iCloud Drive, but because my MacBook Pro is nearly out of local storage, and it has a small 250GB internal HD, I'm now seemingly unable to get even download the iCloud folders due to no local storage avail

I’d start by copying the local documents first and then deleting those files to slowly free up space to continue offloading files. You should be able to download files that are less than your available storage and keep repeating.

You might consider getting an external hard drive and transferring pictures from your Mac to the drive -- without disturbing anything with iCloud.

A friend's phone hand about 7,000 pictures on it. She was getting a new one and that quantity of pictures prevented her from making a backup before setting up the new phone. I backed up the phone first to my iMac, then we transferred the pictures to an external drive - two actually - and deleted them from the phone and my iMac.

Sorry for any confusion - I meant that you could use the same plan to transfer documents (to an external drive) that I used for transferring pictures.