Thanks for your reply @asdffdsa, thanks to your second and third suggestions I managed to fix it!
First, the mount command didn't work on the command line. Sorted that out by changing that ExecStart line to:
ExecStart: rclone --vfs-cache-mode full mount "onedrive": /home/mjt/OneDrive
However, after this the error still remained. Thanks to this Rclone forum post in particular I figured it out. Turns out the issue was that systemctl was running the service as root, while I had initially set up the rclone remote on my regular user account. As explained in the linked post, the root user has a different default config file than a non-sudo user. Adding this to rclonemount.service under the [Service] section fixed it:
[Service]
...
User=mjt
...