You can install OpenWRT on some Fritz boxes. I would still consider replacing it with some better network hardware. Like Ubiquiti.
That said, after a brief consult with Google Bard, evidently QoS on Fritz box is called “prioritization” and is located in Internet → Filters → prioritization. I assume you can de-prioritize traffic from storagenode IP or prioritze traffic from all other IPs.
If you want to rate limit on a container level you probably may use something like GitHub - lukaszlach/docker-tc: 🐳 Docker Traffic Control - network rate limiting, emulating delays, losses, duplicates, corrupts and reorders of network packets using only container labels or a command-line interface.. But router is the right place to manage traffic.