Have you tried the Speedtest using different devices? Gigabyte LAN preferably.

My C1200 LAN recorded 890Mbps using android box H96 Pro+ , a bit faster than my MacBook Pro which gigabyte LAN is based on Thundebolt interface.

Could be the settings in your router, or the fact your router could not handle the full speed.
Nah, it is good, but don’t expect too good of a performance, like those expensive routers.
One advice for you. CHANGE ROUTER. Refer to 1st page, 1st posy, green text. TM should not do anything. It is your router that is the issue. Sounds like your 2.4GHz band, which is the only band available for your router, is congested or full of interference. Refer to 1st page, 1st post, green text, and get a new dual band router, and go on the 5GHz AC band and your problem will be solved.
The Killer NIC on your PC could be the issue. Uninstall Killer Networking Suite, and only get the basic drivers for it.
It is, but Cat 6 is better, and less low quality shit out there compared to Cat5e.

Currently I am using AirPort Extreme connected to TM TPLINK Archer C5 at downstairs. Managed to get around 700 - 1000Mbps 5GHZ wireless AC if I am downstairs. Tested using iPhone X and Mac Mini (via cable to C5).

Upstairs signal is week, so few months back I bought TP LINK power line TL-WPA8630P. With iMac upstairs connected to power line 5GHZ AC can get around 500 - 700Mbps.

TS is of the opinion that power line devices are expensive and sensitive to electrical power disturbance. And it needs to be on the same phase to work.