[noggin]

Are you able to run iPerf taking the Powerline adaptors out of circuit to compare the performance of your uGreen with my Anker GigE/USB 3 solution?  I very much doubt the OTG adaptor is playing any part (though I like mine as the 90 degree moulding is a neater solution than a split cable IMO)

I can try running iperf server on the router indeed for which I'm sure I will get better speed, but at least it can show me whether behaviour of loosing speed is the same.

I'd take your router out of the equation for iPerf. Use a laptop or desktop running on a cabled network connection to your FireTV Stick 4K via a GigE Switch or via your router acting as a switch? (Routers may run out of steam running high bitrate iPerf?)

I doubt otg cable is the culprit myself, but I have doubts about ugreen adapter as its much slower in my laptop as well. Also something fishy about it - when I plug it into otg cable my fire stick looses power and reboots, whereas other USB devices like HDD enclosure doesn't case the reboots...

Could be the UGreen draws too much power on first insertion and drops the power rail from your PSU too much? I always connect my USB GigE+Hub prior to connecting power to avoid this. I don't hot plug stuff like that.

As for powerline adapters - I'm looking at different solutions as my set up was clearly not prepared for 4K times (4 powerline adapters split between three floors and three power circuits). I'm going to try devolo gigagate instead for my main ground to second floor routing and since there are no concrete/bricks involved I'm hoping I can get close to 400-500mpbs that should solve my problem. This is why I'm not yet investing in any other adapters as it could be that local AC WiFi could be enough after that. 

  
Yes - I feel your pain. In another location I have three TPLink Gigabit Powerline adaptors to connect my broadband router, a switch and WiFi access point behind my TV in the living room and a secondary switch in a 1st floor office.

When I move next I'll bite the bullet and put in cabled network connections in every main room...