Could not sign Transaction with SignRawTransactionWithKey through the RPC (RegTest)
You are passing the parameter with signedTx incorrectly. A JSON-RPC param key must have a value that is either an array or an object. You are passing in a string. Instead you should pass in the parameter similarly to how you did for the signrawtransactionwithkey call.

You should have something like this:

     Dictionary&lt;string, object&gt; param = new Dictionary&lt;string, object&gt;()
            {
                {"hexstring", signedTx }
            };

    string decode = await disp.DecodeRawTransactionAsync(param);


The same should be done for sendrawtransaction.