I've noticed one of the resources you listed is the LEADTOOLS SDK. (Disclosure: I'm a LEADTOOLS employee).
Converting to the ePub file format from an input PDF can be done through the use of the DocumentConverter as you have listed https://www.leadtools.com/help/sdk/v21/dh/doxc/documentconverter.html
To test this in Android code, you can try the following:
Uri imageUri = data.getData();
ILeadStream stream = LeadStreamFactory.create(getContentResolver().openInputStream(imageUri),true);
DocumentConverter convert = new DocumentConverter();
ocrEngine = OcrEngineManager.createEngine(OcrEngineType.LEAD);
ocrEngine.startup(null, null, null, OCR_RUNTIME_DIRECTORY);
convert.setOcrEngineInstance(ocrEngine, true);
convert.setDocumentWriterInstance(new DocumentWriter());
File file = new File(this.getFilesDir(), "output.epub");
String path = file.getAbsolutePath();
LEADDocument inputstream = DocumentFactory.loadFromStream(stream, new LoadDocumentOptions());
DocumentConverterJobData jobData = new DocumentConverterJobData();
jobData.setDocument(inputstream);
jobData.setOutputDocumentFileName(path);
jobData.setDocumentFormat(DocumentFormat.PUB);
jobData.setAnnotationsMode(DocumentConverterAnnotationsMode.EMBED);
jobData.setRasterImageBitsPerPixel(0);
jobData.setRasterImageFormat(RasterImageFormat.UNKNOWN);
DocumentConverterJob job = convert.getJobs().createJob(jobData);
convert.getJobs().runJob(job);
Assuming you are passing in a PDF file which is populated through imageUri, this will work. If you would like a starting template to work with, you can try supplying this code inside this template: https://www.leadtools.com/help/sdk/v21/tutorials/viewers/cross-platform/android/display-images-in-an-image-viewer.html