Looking for a Cloud API service or SDK which allows to fill the PDF Form

MuPDF Python SDK comes close but the only problem is it with the checkboxes where it creates a Tick mark instead of Cross mark. In certain forms its mandatory to have those Cross marked instead of tick marked. Other cloud API services provide form fill API but they render the final output pdf - uneditable - which is another problem. Happy to know about paid solutions as well but they should honor the cross mark filling of checkboxes and also keep the form editable like MuPDF does.