Hello,
-verb convertPdfTo only opens the tool "Convert from PDF" in the Toolbox window with the file loaded; -outputType is not used there. For a conversion without a window, use the DocTool with a profile and set the PDF standard (since version 11.31.0):
"C:\Program Files\PDF24\pdf24-DocTool.exe" -applyProfile -profile default/best -profileParam pdf-standard a-1 -silent -outputFile "C:\out\prueba.pdf" "C:\prueba.pdf"
Possible values for pdf-standard include a-1, a-2, a-3, a-4 and a-4f. Alternatively, create a profile of your own with the PDF standard "PDF/A-1" in the PDF24 settings under "Output profiles" and use it with -profile user/<name>. Details: https://creator.pdf24.org/manual/11/#1114-apply-a-profile-to-input-files
Converting to PDF/A also works online: https://tools.pdf24.org/en/pdf-to-pdfa