Their support page does imply if you want to avoid cloud sync you should keep it offline, but perhaps that is just because it is the most brief/user friendly way to describe the situation. There definitely isn't an explicit option to turn off the cloud sync in the device settings, but I wouldn't be surprised if there are workarounds to this once you ssh in. You could also maybe block internet access to the device via your router settings, so you could at least use rsync while at home?
https://support.remarkable.com/hc/en-us/articles/36000264829...