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.
You can download the plugin here.
Follow these steps to install the Smart To Mobile plug-in in Windows (steps on a Mac should be similar, but I have no experience on a Mac):
To update the plugin, follow steps 1 through 3 above, then in File --> Plug-in Manager, select 'Smart To Mobile' and click the "Reload Plug-in" button.
For instructions on using Smart To Mobile, see the Help Page.
Smart To Mobile is provided as-is, with no warranty whatsoever.
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 contact me here.