An android box running stock android tv, with 4K, HDR, half decent memory (2 or 3 gigs), half decent cpu, with the usual codecs (h264, h265, dolbi surround and so on), Netflix, two usb 3.0 ports and one Ethernet port....
Ffs give me a mi box with an Ethernet port and two usb 3.0 ports... I’ll pay extra!!! :( (cries while trying to stream 4K in plex with bit rate bumps nearing the 400mbps and 5ghz WiFi is struggling :( )
Your requirement for Netflix in HD is the limiting factor.
What the heck are you streaming that has bitrate spikes of nearly 400 Mbps? Even 4K blu-rays peak out at 128 Mbps...
What about adding an Ethernet port to the mi box, I bought one of those USB to Ethernet adapters and so far nothings ever had to buffer (at least on a service that can handle the bandwidth). Even 4K Netflix and 4K 60 fps HDR YouTube content loads fairly quickly.
This will use up your only USB port so that might be a deal breaker if you need it for Plex external storage.
Maybe something like this
https://www.amazon.com/TeckNet-Aluminum-Converter-Ultrabooks-Notebooks/dp/B0178HOTOU
The extra USB ports and Ethernet will help but I haven't tried it, should be backwards compatible just your speeds will be reduced to 10/100 on Ethernet and whatever the data rate is on USB 2.0. The theoretical maximum is 480 Mbps but I wouldn't expect anything near that, specially if you are using the Ethernet at the same time.
Sounds like you want a Shield.