You should set TestNet's RPC connection port rather than MainNet's.
It's "18332"
Bingo! thanks!
Now the larger question - is this node (testnet) any good? I want a real node to make real transactions rather than a "sandbox".
Not sure what I clicked on while installing the core. Do I need to start over again and install it as "mainnet"?
Depends on what you want to do... Do you want to learn about bitcoin in a safe way without any chance of losing "real" money, then yes, testnet is perfect... Testnet is an ideal tool for learning, experimenting, scripting,...
If you actually want to transfer value, receive payments, make payments, invest,... you'll need to switch to the main net.
It's basically in the name:
testnet, tBTC (testnet BTC) is worthless, testnet can be reset at any time in the future and all your unspent outputs on the testnet will be gone... Don't buy tBTC, get it from a faucet and use it to learn how to use your wallet, create transactions, spend funds,... Once you're done learning and experimenting, switch to the main net.