After tinkering with my btrfs-microsd and researching about btrfs in general, here's how I decided to handle my btrfs-microsd.

My guide to using btrfs-formatted microsd on Jolla

NOTE: You do this with your own discretion! Something can always go wrong and it may not work for you even though it works for me.

(1.) First you'll want to format your microsd inside Jolla

At this point we have two choices, depending on your needs.

(2.) For basic functions, just use vi to edit one line of the script in mount-sd.sh (located in /usr/sbin/) to be like this:

More info here
With this you'll achieve the basic software support for your sdcard (Sailfish applications find your music/pictures/videos/etc, pay attention to file permissions if they won't). Next move to step 10 if you chose this way.

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Now to the more advanced 'command-lining' to achieve similar results as per the title & OP of this thread.

First do step #1, omit step #2.

(3.) Mount the default 'root' volume of the btrfs.

(4.) Then let's move to the mounted directory for easy subvolume creation.
Now you'll see the device ID of your microsd and can continue:

(5.) Now create subvolumes 'Jolla' and 'Other':
After that we'll create subvolumes inside the 'Jolla' subvolume:
(6.) Then we'll need to modify file permissions of the subvolumes to be the same as their counterparts in /home/nemo to avoid permission conflicts (I had to do factory reset once when these were not right):
PS: You might want to check who owns the /data/sdcard folder in your Jolla and chown that btrfs subvolume accordingly, I can't check it as I don't have one (no Android runtime installed) EDIT: chmod 777 is needed.


(7.) Next we need to find the subvolume ID of the 'Other' subvolume and change it to be the default mounting-point of the btrfs-volume:

For example I found the ID was 269 in my sdcard, so the command was like this in my case:
(8.) Next is the copying of files from /home/nemo to their counterpart-subvolumes:

(9.) Now we get to the final stage, getting the subvolumes to mount to /home/nemo:
(unmount your sdcard first)
Edit the mount-sd.sh to be like this with vi:

Final step. (10.) Reboot your phone.

If you did the advanced command-line editing, you should now be able to copy everything to your sdcard with MTP-mode from your computer, and Sailfish applications should instantly find any music/pictures/videos/etc. from sdcard.

Pros of the approach:
+automount-behaviour
+Easy handling of files via MTP
+Sailfish applications are able to find the contents of your card
+Your phone's internal memory won't get cluttered with files
+'Other' subvolume being mounted by default (more info below)

Cons of the approach:
-Complicated approach
-'Other' subvolume being mounted by default (more info below)

What do these modifications do?

You actually create two 'main' subvolumes of btrs in your microsd. Coupled with the changes to btrfs default mounting point of your microsd and changes to the mount-sd.sh script, This means your Jolla actually mounts two subvolumes of btrfs, 'Other' to /run/user/100000/media/sdcard/ and subvolumes within 'Jolla' subvolume to /home/nemo. What this means is that if you ever happen to mount your microsd to another device, it always mounts the 'Other' subvolume, contents of the 'Jolla' subvolume will be invisible to that device unless you mount the btrfs root subvolume of your microsd like this:

All subvolumes and their contents are visible when the root subvolume is mounted. Note that because Jolla mounts both the 'Other' and 'Jolla' subvolumes, you should avoid keeping duplicate files in them as for example Jolla's default music player searches the 'Other' subvolume as well as Music sub-subvolume (except if you modify the permissions of 'Other' subvolume, which I haven't tried, but it will undoubtedly stop that behaviour if you so wish. It's up to you.)

If you don't like this dual-subvolume setup and prefer to keep it all visible and bundled in the same mounting point, please refer to my links to J4ZZ's and bob_bipbip's posts below (but I'd still change the file permissions of the subvolumes and their mounting folders to be identical with each other before mounting as this at least forced me to do a factory reset when the copying of files via MTP-mode jammed.

bob_bipbip and J4ZZ have also good guides and offer different options when it comes to this topic & btrfs (& I learned a lot from their posts )
Some references:

FUNTOO's great article about btrfs (still valid)
A good guide by Oracle
Guide to basic vim commands

Also, if you have a suggestion to make this guide better and/or more secure, go ahead and make a suggestion.