Hello,
sorry for the late reply. Without the -silent switch, the DocTool reports problems and questions in dialog windows. In a scheduled task such a window is usually not visible, so the process keeps waiting for it and Start-Process -Wait does not return.
Please update to the current version of PDF24 Creator and add -silent to your arguments (available since version 11.31.0). In silent mode the DocTool shows no dialogs, writes errors to stderr and ends with an exit code instead of waiting for input. Your arguments already contain -profile and -outputFile, which silent mode requires. The exit codes are listed in the manual: https://creator.pdf24.org/manual/11/#11119-exit-codes
You can also convert all RTF files with a single call instead of a loop: pass the folder as input file together with -expandDirs, -includeFileExtensions .rtf and -outputDir for the target folder. The options are described here: https://creator.pdf24.org/manual/11/#11-pdf24-doctoolexe
RTF files are converted with Microsoft Word if it is installed. If the task still hangs, please tell us whether a WINWORD.EXE process remains open in the Task Manager and whether the task runs only while you are logged on or also without a logged-on user.