Hello. I'm planning to write a somewhat complex system and I'm looking for advices about software that could help me achieve that.

There will be clients connected to a hub in house. Hub's will be connected to a Remote Server that would store time series telemetry from clients. Hub's and the server would contact with each other, server sends the information about tasks hub's should do (based on what the user clicked on the web), and hub's return the result.

Clients and Hub's will be behind NAT so I need some framework that would allow event driven connection between hubs and the server. (hubs would register to the server, and there should be callback functionality in the hubs). That excludes Akka.net(?).

Server should also provide dashboard with statistics of every client (using ChartJS). Because there will be a lot of data from clients, it has to be cached or reduced before storing to database otherwise queries would take minutes. So Spark SQL?

Client and Hub software will be written in C#, Server will be .NET Core based website.

I've already chosen Cassandra (ScyllaDB) as a database for time series, for other things like user details etc. I'll use MySQL Im looking for suggestions about frameworks I could use to achieve the goal. Any help is appreciated.