A quick update on my progress, pretending that there is someone interested 
OneDrive migration: Rclone did its thing, but the external drive that I cloned everything too is exceedingly flaky, and keeps disconnecting / reconnecting from my. So I painstakingly copied everything from the flaky drive to a brand new 4tb external drive using rsync, and the new drive is now my ‘main’ drive.
I only messed up once and unplugged the new drive early, messing up the file system tables, but I fixed that and kept going. 
And once the copy was complete, I made another rsync copy to a second brand new 4tb external drive, verified that it worked, and this is my backup. It’s now hiding in another room. Not ‘off site’, but at least disconnected.
My current dilemma is settling on the best way to connect my main drive to the network. I tried plugging it straight into my router and setting up an SMB share through the router software. I can connect fine from my LMDE laptop, but not from my Windows laptop. Well, I can connect from Windows but not read/write files consistently. I’m getting errors about connecting via SMB 1.0 because that’s all the router supports. Ugh.
So now, the drive is connected to the LMDE laptop directly. Works a treat via linux and I can ssh in from Windows Terminal no problem. But I still might set up my raspberry pi as a NAS and access it remotely.
I haven’t made any real progress on organizing the mounds of data on the drive, other than a few sessions of moving old files around and deleting the most obvious duplicates and junk. So it’s quite early days for this project.
And I haven’t got the nerve to ditch windows completely and install linux on my main laptop. Soon. I think that will actually simplify things quite a bit.