Expanding the capacity of even the best NAS is simple. Fill the enclosure with as many drives as it supports, and add an external enclosure once the primary system is fully populated. Larger drives can replace smaller capacities as they reach their end of life, but there’s also the possibility of using USB drives to expand capacity. Whether or not you should do this is a different matter altogether.
In my own case, I do actually use the USB interfaces on my home server. But I have USB-SATA connectors going to each 4 TB hard drive. At 2am every morning, the server does an incremental rsync backup from drive 1 to drive 2. So I don’t sit with the drives running all day long in a RAID configuration. For me, it’s working fine right now.