You could try checking the "Attempt to support newer firmware" option in the "Metadata, on device & advanced" tab of the KoboTouchExtended plugin so it might be fine. I don't think I've seen a warning against doing otherwise in the last few years, but you might want to ensure that you're running the last version supported by Calibre 3 first (according to my installation that I keep around on an older Intel Cherry Trail machine, it's
3.4.4). But Calibre 3 may not have a Sage driver and if that's the case, then you're SOL. But it's worth a try.
A little advanced, but for a free solution, you could try installing
Virtualbox and installing a Linux virtual machine (
Ubuntu might be the easiest), install the Linux version of Calibre 5 on that, then enable USB passthrough and connect your Sage that way. You'll need a lot of free disk space though since it's an entire OS worth of data, (i.e maybe 5GB+ plus space for books).