4k buffering update:
Okay, I am now convinced there are major problems with the bitrate/codec in prime video specifically on the fire tv stick 4k or the ram is not sufficient for instant 4k streaming. I would go with the former, as Passengers( a movie I own) plays instantly in 4k. However, it struggles with any 4k hdr prime content. I am getting 350mbps download on the fire tv stick, which is enough for 8k streaming. However, Amazon sent me this:
This is in regards to 4K content on the Firetv. Our specialists say "I can see fluctuating bandwidth on the device. Please ask the customer connect with the ISP for stable network.
Irrespective of high speed, the customer would require to have a stable connection.
Also, confirm if the customer can try a wired connection on the device.
Note- The video playback will start with a lower quality and increase as per the bandwidth."
It appears they are seeing something very unstable, and it's clearly something in the app. I think I will try to work with Amazon a couple more times as I've been inquiring about this for months, but it seems the best bet for amazon content is to buy a third party device or maybe try an amazon device with better ram(the new cube).