Seeking Guidance for Lightroom Sync and Storage Management as a Digital Nomad Amateur Photographer

If I may, I would like to make some comments on the statements in your post.

Using correct terminology and App names is important to understanding questions.

What is your 'Primary' photo editing App?  Lightroom or Lightroom-Classic?

COMPARE LIGHTROOM & LIGHTROOM-CLASSIC FEATURES

"Photos are imported into Lightroom from the sub-folder on my Mac, creating a new catalog."

Lightroom or Lightroom-Classic?

As stated by @dj_paige multiple LrC catalogs is "not a good idea"! For a start you can only use Lightroom-Cloud syncing with one LrC catalog.  SYNC SETUP IN CLASSIC 

"The newly imported photos are then synced in the cloud via the Photography plan"

So How do you do this- specifically your workflow?

If you are using the {Creative Cloud} Folder in Mac Finder- that is a problem! This use of storage will cease on 1st Feb 2024.

LrC requires the creation of  Collections that are marked to sync. Do you do this?

"My Mac's "Pictures" folder is 114GB, while the space occupied in the Lightroom cloud is only 3.7GB."

Not unusual, IF you are syncing photos from LrC 'up' to the Lightroom-Cloud.  LrC does not sync full-size files. Instead it sends proxy files (smart previews) that do not occupy any of your allocated Cloud space.

"I've noticed a "Mobile Downloads.lrdata" folder using 10GB, even though I don't use Lightroom on mobile or tablet."

Have you used Lightroom on your Mac to Import/Add photos?  This WILL move the full-size files to the Lightroom-Cloud occupying space.

Have you used the {Creative Cloud} Folder in Mac Finder to 'store' files? This WILL occupy space in your 20GB allocation.

"Am I inadvertently using more space than necessary on my Mac with the current setup?"

If Lightroom-Classic is to be your 'primary' editing app that controls your precious photo files, then it MUST store ALL your files 'locally'. Either on internal drives, or external drives. And file backups need be made locally.

"Considering my circumstances, should I manually create a second copy on an external drive or another cloud for added security?"

"External Drive"- YES! Using the Adobe 'Cloud' is not a backup method!

"As someone with a basic photography workflow, how can I ensure the security and accessibility of my photos, especially in the event of losing my Mac?"

Backup your photo files and the LrC Catalog to other locally attached drives.

"simplicity and security in my photography journey."

My suggestions-

1) Make Lightroom-Classic your primary photo editor and cataloguing system.

2) Store and make backups of all photo files and the catalog on local drives (physically different to the working catalog drive).

3) Create Collections in LrC that sync (proxy) photos to Lightroom-Cloud

4) Use Lightroom when travelling.  (Noting that imports to Lightroom will use Cloud storage AND download these files to your LrC Desktop folders. There are methods to then reduce the used Cloud storage.)

Hopefully my advice might help your "journey".

Creating a new catalog for each import, generally not a good idea. You are fragmenting your ability to search across your photos; and a gazillion other disadvantages as well.

Hello. Regarding your workflow overwiew 3,4 and 5 can be done at the same time when you import in LrC. So this could be simplified by using more lightroom. You import automatically from you Card  in Date folder, and you use keyword for example. 

When you synchronize from LrC to the cloud only Smart preview are send and they are not counted in your 20GB Quota. This explains the difference

And your last concern is about Backup and the good solution depend on what you are ready to lose. It can go to a copy on a external hard drive to some 3,2,1 strategies with a NAS synchronizing automaticly on the cloud. It is what I have and I'm not a pro 😉 

Hope it helps