Lightroom as Main Library but utilizing Lightroom Classic to edit GPS data

Yes you can do that, but there are a few "gotchas" that you'd need to be aware of:

1. When you enable sync in the LrClassic (LrC) catalog a copy of ALL the images that you have in the cloud will download into the Classic catalog and will be stored on a local hard drive. You can control the location of those locally stored images, and even choose to have them stored in a standard date-based folder scheme, but best to set that location BEFORE you enable sync. The options to change location and choose folder scheme are to be found on the LrClassic Preferences>Lightroom Sync tab.

2. Once the images have been synced you can use the standard Library Filter in LrC to find those images that do not have GPS coordinates assigned, no need for a Smart Collection. If you then add GPS coordinates (either manually or by using the LrC Map module), those coordinates WILL sync back to the original image in the cloud. Be aware, though, that if you add Location data (Sublocation/City/State/Country) in LrC, that data does NOT sync to the cloud. So if the cloud is your master application, best to add location data in Lightroom desktop (though once the GPS coordinates are added, Lightroom should automatically try to do a reverse lookup to add location data - though that may not always be as you might prefer).

Provided you have the local storage space, using LrC in this way is in my opinion a better backup solution than using the option to "Store a copy of all originals" on the Lightroom Preferences>Local Storage tab. Maintain the discipline of always importing new images to any of the Lightroom apps, not LrC, and it should all work out OK. But don't be tempted to add Keywords in LrC, because they also don't sync back to the cloud.