I have an external drive with three partitions. In one user account, I like to have one partition mounted. From that... drive (or mount), I bind folders into my Home folder.
The partition is never available to service type applications until I manually go into that item with a file explorer.
Is there something I can put into the fstab file? Likewise, my bound folders don't show up until a re-run the mount command to mount/bind those. Is that something that should be in the fstab file as well?
If you look in Disks, and look at the mount options for the drive you're looking at, it might LOOK like the automount is already on, but you actually have to turn off "Automatic" for the drive mounting, then you can manually toggle the option for it to be available right away.
I've run into that before where I see the toggle as "enabled" (paraphrasing, not at my home pc right now) but it's actually not because the system is doing its thing "automatically".
This article shows what I'm talking about more in depth:
Auto mount disks with GNOME disk utility | PikaOS Linux
You have to DISABLE "User Session Defaults" and then you can have the checkbox for system startup enabled. Confusing (imo) for the majority of users.
Apple beat me to it, and no doubt his solution will work, since he's already helped me, but this automount worked for me, following instructions in a YouTube video. In the screenshot below, my logical volume is called LV800.
Disks/click on the disk in question/tools icon/Edit mount options/User defaults off/Mount point disk name/Identify as disk name.
This worked for me, so I'm just passing it on.
As far as this part of the question (which I admit should be another topic), I decided to take a different approach. I edited my ~/.config/user-dirs.dirs file to 'point' to the specific directories that are on the different drive and then just put links to those folders in the home folder. So that config file looks something like this:
XDG_DESKTOP_DIR="$HOME/Desktop"
XDG_DOWNLOAD_DIR="$HOME/Downloads"
XDG_TEMPLATES_DIR="$HOME/Templates"
XDG_PUBLICSHARE_DIR="$HOME/Public"
XDG_DOCUMENTS_DIR="/media/DataPart1/OneDrive/Documents"
XDG_MUSIC_DIR="/media/DataPart1/OneDrive/Music"
XDG_PICTURES_DIR="/media/DataPart1/OneDrive/Pictures"
XDG_VIDEOS_DIR="/media/DataPart1/OneDrive/Videos"
XDG_SOURCE_DIR="$HOME/Source"
XDG_SOURCE_DIR="/media/DataPart1/OneDrive"
tl;dr
This feels more like Windows where those familiar file groups/directories can be set to point anywhere and that's usually OneDrive. So, now w/ InSync handling the sync from OneDrive albeit to a larger hard disk drive while the OS is on a smaller solid-state drive, there is more of an integrated feel, and files are backed up to the cloud and then there is an external hard drive handling backups.
Another comment:
Ever since I've done this, Nautilus is much more stable now. Before, I went in and messed w/ the folder configuration, two out of three or even more times that I would go to use Nautilus, it would just give me the 'Wait or Force Quit' dialog.
I wish I knew and could say decisively what and if it was something I did. #noguesses ! 
After some use since posting this, I retract what I said about Nautilus being more stable. It seems to be just as buggy as before when I first installed 18.
Thunar is always working well, but I do like Nautilus better, when it works.
Have you tried Nemo? I tried a dozen file managers, including Thunar & Nautilus, and Nemo is the only one I've found that works for me. Permanent dual pane (if wanted), drag & drop, easy file renaming - the only thing that I'd like added is the retention of last position, as it always opens both panes in the home directory.
That and ordering per directory would be nice. I'm never sure if that is a kernel, distro, Gnome, or file explorer thing as it seems to be the norm for any file explorer I've used in Zorin OS.