One limitation of Adobe Photoshop Lightroom Classic CC is that it cannot sync a Smart Collection to Lightroom Mobile. Smart To Mobile is a Lightroom plug-in which syncs a Smart Collection into a normal Collection which then can then be synced to Lightroom Mobile using the techniques built-into Lightroom. Even better, you can specify an entire set of Smart Collections and their corresponding normal Collections which you can subsequently sync with a single button-click, letting you update Lightroom Mobile with the changing contents of your Smart Collections (by way of the normal Collections which Smart To Mobile updates for you). And don't worry, you needn't be concerned about throwing away disk space with these extra collections since both Smart Collections and normal Collections are really just lists of pointers to the one-and-only copy of your photos within your Folders.
For download, installation, and version history of Smart To Mobile, please see Smart To Mobile's home page.
Here's the way I use Smart To Mobile. I have a bunch of Smart Collections that I wish Lightroom would sync to Lightroom Mobile. I create a Collection Set named "For Lightroom Mobile" (any name will work fine), then inside that Collection Set I create normal Collections with the same names as the Smart Collections I'd like to sync. Actually, I put the word "Collection" at the end of each of those names so it'll be clear to me that these aren't my Smart Collections. This naming convention is just my choice; any names will work just fine. I leave these collections empty (it's Smart To Mobile's job to populate them with photos), and I mark them to sync with Lightroom Mobile by right-clicking each one and being sure "Sync with Lightroom CC" is checked. Then I collapse the "For Lightroom Mobile" collection set and never look inside it again. Then I run Smart To Mobile to configure it with the list of Smart Collections and their corresponding normal Collections (the ones inside my "For Lightroom Mobile" Collection Set). I click the "Sync All Remembered Collections" button to copy the photos from my Smart Collections into their corresponding normal Collections. Remember, this doesn't actually make a copy of any photo; Lightroom just points the Collections to the same photos that the Smart Collections point to. Finally, once a week or so, I run Smart To Mobile again and click the "Sync All Remembered Collections" button to update all the normal Collections with the potentially-updated contents of the Smart Collections.
Open the Smart To Mobile dialog by clicking the Smart To Mobile menu entry on the Plug-in Extras menu of Lightroom's File menu or Library menu. The remainder of this page describes each control on that dialog.
Delimiter: Start by opening the 'Target collection' combo box list by clicking the downward-arrow at its right edge. You'll find each of your Collections listed, with a hierarchical name that shows the full path through your Collection Sets down to each Collection. A character, called the 'Delimiter' is placed between each part of that hierarchical name, just like the backslash that is used between folder names and file names in Windows. It is important that this delimiter character is not used in any of your Collection, Collection Set, or Smart Collection names, otherwise the hierarchical names cannot be properly interpreted. The first time you run Smart To Mobile it will select a delimiter character for you which doesn't appear in any of your existing names (unless none of the 12 characters it checks are unique, in which case it will warn you with an error dialog). You can change the Delimiter character if you'd prefer a different one, just as long as you pick a character not used in any of your names.
Smart collection: This combo box lists all the Smart Collections in your Lightroom catalog. Select the one you'd like to sync into a normal collection.
Target collection: This combo box lists all the normal Collections in your Lightroom catalog. Select the one you'd like to sync that Smart Collection into. (For me, that's one of the Collections in my "For Lightroom Mobile" Collection Set.)
Mode: The Sync button below and the one near the bottom of the dialog are the only buttons that can do anything to your Lightroom Catalog. You're probably worried that Smart To Mobile is going to party all over your catalog in ways you don't expect. Relax, because Test Mode will not ever make any changes to your catalog. Instead you'll just see a dialog with all the changes Smart To Mobile would have made, had it been run in Update Mode. So please run your first Sync or two in Test Mode to gain confidence in the plug-in and understand what it will do. Then, once you're comfortable, switch to Update Mode. Syncing in Update Mode will actually update the Target collection, as described in the next paragraph.
Sync Smart Collection to Target Collection: This button (enabled once you select both a Smart collection and a Target collection) will compare the file list of the two collections. Any photos in the Target collection which are no longer in the Smart collection will be removed from the Target collection. Then any photos which are in the Smart collection but not in the Target collection will be added to the back end of the Target collection. It's up to you to manually sort or rearrange the photos in Target collection once Smart To Mobile has put your photos into it if you wish for them to be in some particular order.
Remember This Synchronization: Here's how you build a list of synchronizations you'd like to do over and over again. Once you've selected a Smart collection and a Target collection, this button will become enabled. Clicking it will add that Smart collection and that Target collection to a list of collections that will be remembered, located below.
Remembered target collections: This combo box holds a list of the Smart collection / Target collection pairs which you've remembered. Actually, only the Target collection names appear in the combo box, but the Smart collection is also remembered behind the scenes. The reason for this list is so you can re-synchronize all of these pairs of collections in the future, once photos have been added to or removed from the Smart collections. You do so using the "Sync All Remembered Collections" button below. When you click an entry in this combo box, the Smart collection and Target collection combo boxes above will be updated with the data in that remembered entry, so you can see both Collection names.
Forget This Remembered Sync: If you want to remove an entry from your Remembered target collections, just select it within the "Remembered target collections" combo box, then click this button. It will remove that entry from the remembered list of Smart collections and Target collections so the "Sync All Remembered Collections" button won't update that Target collection any more.
Sync All Remembered Collections: This button is enabled whenever you have any entries in your Remembered target collections. Clicking it will go through that remembered list, one at a time, and sync each Smart Collection into its corresponding Target Collections. Once you've setup your collections in the top half of the dialog, this is the button you'll click every week or two to update your target collections. Lightroom will then in turn sync those Collections to Lightroom Mobile so you can show your friends or clients the photos in your Smart Collections.
Help: Clicking this button will display this help file in your web browser.
Close: This button closes the Smart To Mobile dialog and returns you to your normally-scheduled Lightroom workflow.
Smart To Mobile is provided as-is, with no warranty whatsoever.
For clarity, here are a few things you should keep in mind.
If you use Smart To Mobile frequently and find it helpful, please donate whatever you feel it's worth to my Paypal account at paypal.me/JeffBondono. I spent about 32 hours writing and testing this plug-in and it's help file, so a few dollars from a hundred of you would be great. Somebody contributing $100,000 would be even greater. But seriously, I appreciate any donation you might care to make, and it will encourage me to maintain and enhance the plug-in as needed, and write more plug-ins in the future.
You can send me an email.