Hello, there. If you have trouble transferring files from one Mac to another, why not give the disk clone a try? The disk clone function has the same effect as Migration Assistant does. You can use this function to grab everything on your disk and then transfer the cloned files to the target disk or new machine.
iBoysoft DiskGeeker does good in disk clone and wins all kinds of reviews, you can be assured to free download and install it on your old Mac to complete the files and applications clone task with an external hard drive connected. Once the clone ends, connect the external hard drive to your new machine and start to transfer the clone files and folders.