You did not show the code in SendCommandSynchrously() method; nor you mentioned the code uses AutoCAD COM API or .NET API.
Also, it looks like the LISP command runs a script, which can be macros of multiple commands, or be one or more LISP command in it. So, it is very difficult to say for sure what kind of output/result of the script execution would be.
On the other hand, if you want to call a pure LISP function that returns some data (not all LISP functions are designed to return some data other than nil), you can use .NET API to run the LIST function:
ResultBuffer result = Application.Invoke([Input ResultBuffer]);
where the [input ResultBuffer] include the lisp function name, and expected inputs required by the lisp function. Of course, the LISP function should have been pre-loaded.