Reading pixel data from Texture2D on GPU

Hi,
I need to read pixel data from a Texture2d, each frame, that I need to pass to an enconding mp4 plugin.
Unfortunately texture data is on GPU only, therefore I cannot simply use GetPixels32().
What's the best way, suitable for 25 fps streaming that won't cause big cpu/gpu spikes?
At the moment I do this, but I'm not sure it's the best method:

Code (CSharp):

1. List<AsyncGPUReadbackRequest> requests = new List<AsyncGPUReadbackRequest>();
2. void Update()
3. {
4.   var request = AsyncGPUReadback.Request(textureToEncode);
5.   requests.Add(request);
6.   for (int i = 0; i < requests.Count; i++)
7.   {
8.     AsyncGPUReadbackRequest r = requests[i];
9.     if (r.done)
10.     {
11.       videoRecorder.CommitFrame(r.GetData<byte>().ToArray(), clock.Timestamp);//ToArray() is bad
12.       requests.RemoveAt(i);//resize maybe could be optimized?
13.       break;
14.     }
15.   }
16. }

Any suggestions?
Also not all devices support asynchronous readback so it would be better to go for another way as fallback. Which one?I think a small cpu stall would be reasonable..