Easiest Solution imo:

• get Wildcard DNS, point it to the public IP of your NAS
• deploy the ssl cert (containing your main domain and sudomains for your docker containers)
• configure reverse Proxy in Synology configy proxying requests for the subdomains to your docker container (you can enforce only local access to certain services too)
• Static route or local dns (Pihole) to redirect local requests for your public ip to the private IP of your NAS
• done!