--rpcuser setting is not what I need, right?
That is correct,
-rpcuser is completely unrelated to what you want to do.
You can start bitcoind with the
-maxconnections=<n> option where is the maximum number of connections that you want to allow (or add the option to the bitcoin.conf file).
Then you can setup an external firewall to manage your connections. With that firewall, you allow through n-8-1 external connections on port 8333. This means that you are allowing other nodes to connect to you, but still keep one unused connection less than your
maxconnections setting for your SPV wallet. Then you add another rule to the firewall to allow 1 connection on a different port which just forwards traffic to port 8333. You connect your SPV wallet to that other port.