No, that is not the case, storage services are just shut down during the start of the actual expansion process, for a minute or two. Services are started right after (While to pool is optimizing, storage and apps should be available , done it many times)
Your NAS does not support QuTS, and QuTS DOES support expansion by swapping or adding vdevs in the pool (irrelevant on your NAS though)
Unless you go with a big Core/Ultra Xeon NAS, all NAS use BGA processors, no sockets
Again, swapping disks does not stop any services, unclear what is happening on your NAS (hence the request to open a ticket)