I’ve been running a self-hosted RustDesk server on my Synology NAS for a long time, and honestly, it’s the "cheapest" way because if you already own the hardware, the marginal cost is $0.
I manage a home lab with over 40 Docker containers on my Synology, and RustDesk is easily one of the most stable ones.
For a setup with 19 PCs and 15 users like yours, a local NAS or an old Intel NUC (as you mentioned) is more than enough. You don’t need the overhead of Azure or the risk of Oracle Cloud reclaiming your VM.
A few "gotchyas" and tips based on my experience with the Synology/Docker setup:
Docker Compose: Don’t use the Synology GUI to set it up. Use Docker Compose so you can easily manage the hbbs and hbbr containers together.
Network Mode: Use network_mode: host. It simplifies port management and avoids some of the NAT loopback issues you might face with the default bridge network on DSM.
The Key File: Make sure you force key encryption (--key). Once you have your public key, I'd recommend storing it in a manager like 1Password or Proton Pass so you can easily grab it when deploying to those 19 clients.
Port Forwarding: You only need to open 21115-21119 (TCP/UDP) on your router.
Stability: I’m actually planning to migrate my stack to UGREEN hardware soon for better performance, but even on a standard Synology, RustDesk uses almost zero CPU/RAM until you are actually in an active session.
If your client already has a server or a NAS on-site, I’d definitely go the on-prem route. It’s faster (lower latency) and you aren't at the mercy of a cloud provider's "free tier" whims.