Tile seamless works by introducing some blur. So it doesn't work well in this case. Do it without using tile seamless.
1. Use smooth palette to make a pattern like (1) below. Say 256x256
2. Then use this image to an image like (2) below. Also 256x256. This image is seamless.
3. Tile to much larger than needed. I haven't tested, but probably something like 6000x6000
4. Rotate this large image 45 degrees
5. Crop a 3600x3600 from the centre