If you are in need to get latest Calibre - here are a few options:
i. install VirtualBox + good old Windows 7 - that guest OS works amazingly fast on High Sierra. With folder sharing enabled between your host (MAC) and guest OS - it's easy to manipulate with your files.
ii. Buy Raspberry Pi for ~$35 and install raspbian + calibre there. Maybe read
this. As a result - you will have your library in a tiny form-factor you can grab and go. With VNC enabled feature you can use MAC's internal VNC-like client to connect to your RPi.