Is 16GB of RAM overkill for a Home Assistant NUC running Docker?

Not overkill. Go for it. 8GB is fine for Home Assistant and a couple containers today, but Docker means “a few containers” will become “more containers” within weeks. HA itself can eat 2-3GB with cameras or integrations, and adding Zigbee2MQTT, a database, or a small media server pushes you against 8GB. A 16GB kit costs $30-40 and buys headroom so you don’t have to mess with it again.