You need to do a check of speeds from your internal network.. and then to your external connection.
Degradation in speed is not that uncommon but tracking the exact reason for it is not simple.
So what I would recommend is firstly to carefully test wireless speeds from your laptop to another computer in your local network connected by ethernet to the AE and with it's wireless off.. to eliminate a double wireless hop.
The correct tool is called iperf and was included in command line on older OS.. you will need to check for it in your current OS.. but there are other tools as well which you might find easier to run.
Let me check what other network tools are available. But you might already have something suitable to produce the long term speed test you have done which is really neat.. btw.
You use this in combo with the wireless diagnostics on your computer to make sure your TC is transmitting signal at the best frequency and full 80mhz of bandwidth.
Once you establish the best possible speed in local network then you will have a much better idea of what is going wrong.
Wireless connections are based on signal strength NOT speed.. so in my network to get wireless working to peak efficiency I have to make sure I get a link speed of 1300Mbps.. not two stream 867Mbps.. it is easy to lose speed from even a single wall blocking signal.. AC wireless in 5ghz declines rapidly with distance.
Hold down the option key when you click the wireless fan in the top menu will also give you a quick view of the connection.
This is very close to the Apple TC.. as you can see from signal at -34dBm.. ie very high.
It is on full 80mhz and yet right now I can only get 878Mbps link speed.. it should be 1300 or close to it.

You can also get link speed from the TC.. remember this is two way.
Speed can vary at both ends.
So in the airport utility click the icon of the AE.. and then roll the mouse pointer over the link.. an extra menu item will pop up that shows the connection.

So data rate at the TC (same as your AE).. is 1053Mbps.. which is very good but still not peak.
You can then do a simple test like copying a file to client.. it should be something that has a proper SSD that will not be limited by disk speed.
If I do a speed test by copying a single very large file to/from a fast NAS, I get the following.

The peak there is 80MByte/s so that is 640Mbps.. when you consider the overheads in wireless connection.. that is really good.. it does take a while to ramp up the speed which is also typical so the overall transfer speed is a lot less than its peak.
Run those sort of tests and you will quickly build up a picture of where the choke point is in your setup.