You can ssh into the remarkable and copy files via scp.
I use RCU [1] for that.
0: https://remarkable.jms1.info/info/filesystem.html
1: https://www.davisr.me/projects/rcu/The workflow is plug -> web browser -> remarkable IP -> drag and drop.
Anyways, I think I could have dealt with it if it handled large books fine.
The web interface also has a couple of other advantages: the tablet simultaneously listens for ssh connections, and can be used over Wi-Fi, IIRC? Though it could also expose a "USB HUB" with both the network interface and block storage.
I just wish we had a more ubiquitous "network file storage" protocol. The tablet itself could offer NFS, but mounting it under different operating systems would be a pain, requiring manual user intervention.