Do photos need to be in a collection to sync to Lightroom CC?

There are two methods for synchronizing images from Lightroom Classic CC. The one method involves creating collections, and this synchronizes smart previews with Lightroom CC. The big advantage is that these collections do not impact your cloud storage allotment. That means you have the capability of synchronizing an infinite number of collections of these smart previews in this manner.

The other method involves migrating your entire catalog. This process will migrate all your images to the cloud. Doing this will send full-sized images to the that will impact your cloud storage allotment. You will have to determine if you have enough storage to accommodate your entire 26,000 image library. If you have the basic photography package then you have 20 GB of cloud storage available. Or, perhaps you have purchased additional cloud storage space, in which case you would probably have a minimum of a terabyte of cloud storage. After you migrate a catalog to Lightroom CC, you would then want to do all your importing using Lightroom CC rather than Classic.

Not knowing what plan you have, I can't make any further recommendation.