I have been thinking about your situation. There is probably no perfect situation.
So…. Why make the situation so complicated.
I see huge benefits to you in terms of using Lightroom, but the cost is the initial effort involved. As time goes by the benefits will increase. But ignore Lightroom for the moment.
Assume you have your main workstation in the US and you travel with a laptop to Switzerland.
Why not simply get a second 18TB WD (call it the Travel Archive) and sync this drive from your WS Master Drive. Leave the master drive in the US and bring the Travel Archive with you when travelling. You therefore know you always have your master drive at home and you have full versions of all your images when travelling. Having multiple sub disk copies of your master disk is a potential exercise in frustration and extremely error prone. The extra weight of a single Travel 18TB drive must be similar to bringing a collection of smaller drives.
I would refine this by getting a fast external SSD (say 2TB). Use this (plus a fast port) to store your catalog. When you travel, you can thus plug this into your laptop and you also have your full catalog and all the catalog previews.
While in the US…. add any new photos / edits to your Master WD drive. While you are travelling (ie not only Switzerland) add any new images to your desired folder structure on the external SSD. You can then add these images to your Lightroom Catalog (if using Lightroom). If not using Lightroom, follow your normal process, but add any new content to the external SSD.
When you get home transfer the new content to your Master WD. When you have that completed, sync copy your Master WD to your Travel Archive… so it is ready for your next trip. Any new images /edits while in the US get saved to the Master WD Drive. Just remember to synch the Master WD drive to the Travel drive in advance of any travel.
At home, make sure your Master WD drive has a letter in the mid section of the alphabet (say P for Photos). Allocate the WD Travel to say a letter of Q (ie P & Q are a logical pair). When you travel, and from your laptop, with the Q drive connected, rename the Q drive back to P. This means, if using Lightroom Classic on your laptop, Lightroom will be able to find all the images on the drive. [This may not make sense to you now if you are not familiar with Lightroom and the handling of drives.]
VIP. In this scenario… Do No Import Images From Your Travel WD Drive (P when used with your laptop and Q when used on your main PC). You do not want to create a scenario where you are importing duplicate images to your Lightroom Catalog.
I see lots of pros for using the Cloud Version of Lightroom… and maybe this is a future option, but while you are starting… regard the US as base and use LR Classic to manage your Master WD images and get to base camp. While learning Lr I would not recommend mixing Lr Web Cloud and Lr Classic, especially when dealing with a large volume of images.
As you get familiar with Lr.. you should be able to categorise your 5 star images (or your absolute keepers), add these to multiple and overlapping collections as required. Your workflow will improve as you get more familiar with Lightroom.
I intend to use a modified version of this approach myself going forward. I have my Lr Catalog moved to an External SSD drive and I also keep my last 2 years images on this SSD drive. When I travel, I use LR Classic on the laptop, with my Catalog, All Previews and the last 2 years images on the SSD (now connected to my laptop). Any new travel images get added to their appropriate folder (Organised by Year and Project in chronological sequence) on the SSD. I can edit, add metadata, create jpgs (or PSDs, or Tiffs) as needed during travel and all of this effort is instantly available when I plug the SSD into my main workstation when home. In my case I have no need, while travelling to go back to images captured back in say 2002.
Every January, I move the oldest year on my SSD drive to my internal spinning disk, which holds all my images back to 2002 and open a new folder for the next year (ie 2025).
You still have all options available for Cloud Based Backups. I would not regard cloud synch services, such as DropBox, OneDrive, iCloud, Adobe Web / Cloud as backup services.