streaming compressed bytes to s3

I was looking at different ways to compress and upload a file to s3, and I recently came across the CompressionStream api. Basically uploading a file in one go using this seems to be quite straightforward:

const compressedStream = file.stream().pipeThrough(new CompressionStream('gzip'));
response = await fetch(presignedUrl,{
  method: "PUT",
  body: body,
  headers: {
    "Content-Type": contentType,
    "Content-Encoding": "gzip"
  },
});

Since, I target large files (1-3 gb), I was going for a Multipart upload. But, the Compression stream being a stream api can pipe through data as far as I understand(hopefully correctly) .

So, I wanted to combine the advantages of both, and knowing s3 doesn’t support directly streamed uploads, I wanted to upload the chunked bytes on a multipart upload instead.
Yet, I’m not being able to figure out how to do this, it may look something like this:

// Here, I've tried to use TransformStream but , a better approach is really welcomed
// this is pseudocode only
// file = event.target.files[0] , user selected file:
  file
  .stream()
  .pipeThrough(new CompressionStream('gzip'))
  .pipeThrough(new TransformStream({
    start(){},
    transform(chunk, controller) {
           
      uploadPromises.push(
        s3Client
          .send(
            new UploadPartCommand({
              Bucket: bucketName,
              Key: key,
              UploadId: uploadId,
              Body: some_chunk_of_5mb_size, // THIS IS THE CONFUSION
              PartNumber: i + 1,
            }),
          )
     
    },

   }))

1. What I do not understand is how to get a chunk of size >=5mb , since
   that’s s3’s requirement for a multipart upload.
2. What is the data type of this chunk even? In the Transform stream docs, it’s being compared to all sort of data type, can I even check the size and concatenate this chunk to make it 5mb for the multipart upload?
3. Does uploading like this if the chunk has to be for example further converted into buffer or something affect the integrity of the file being uploaded?