I'd be interested to know why you say Ethernet adaptor is not a good option for Firestick as I have had a similar problem with my bedroom Firestick and going back to Wifi over Ethernet seems to have improved the buffering?
I did test some Ethernet adaptors and I can not get full speed since. Almost of them only reach to under 100 Mpbs even in some case you only get 30 - 40 but with wifi (now lastest Firestick with Wifi 6), you can reach 1000 Mbps.
So I don't need to spend money to Ethernet adaptors to lower my speed