Forget about trying to have Lightroom Classic and Lightroom using the same files on the NAS, that simply can't work without risk of serious complications. The far simpler solution is to sync as many images as you want (all of them if you prefer) FROM Classic to the Adobe cloud. They would be uploaded as smart previews, not full originals, to the cloud and from there they are available on any of your mobile devices. Being smart previews, they do not count against your 20GB cloud space allowance. Smart previews are generally good enough for browsing and some editing on the mobile device.
However, the cloud does not recognise whatever folder structure you are using in Classic, instead organisation is done by using Albums (equivalent to Collections in Classic), and these do sync to each other. Collection Sets do not sync however, so you would have to manually create the equivalent of them in the cloud (which are confusingly called Folders there). Various other things also do not sync between Classic and the cloud, notably keywords and location data. However, if all you want to do on your iPad is browse and edit, doing it the way I've described should work fine. Plenty of other Classic users are using that workflow.
One other thing....if you particularly wanted to organise in the cloud the same way as your images are currently organised by folders in Classic, you can achieve most of that by using the feature in Classic to create and populate a set of collections which copy the folder structure and the contents. You can then sync the collections to the cloud where they would appear as albums. You'd still need to manually create the "folders" in Lightroom though to mimic teh Collection Sets created in Classic.
If you want to import images to your iPad when in the field, you can do that but those images would be uploaded to the cloud as full originals (which do count against your cloud space allowance), and from there they would download into Classic where they would be stored in the location that you'd need to set in the Classic Preferences>Lightroom Sync tab. You could, of course, specify your NAS drive for those downloaded originals and have them placed in the same structure as your main library if you use a date-based folder structure.