The video quality depends on the source quality. If you have media player with HDMI output, you get the picture quality of the media player. If you use the build-in player (or decoder) inside the TV, you get the quality of the video processor inside the TV. It is not about the USB or HDMI in this case. The 4K video streaming need 25Mbps for Netflix. Some would require slightly less or slightly more depending on the color depth of the source data. They probably reduce to depth to to be able to stream more channels simultaneously at standard defined by them. So, even USB2 can transfer fast enough. In theory, it is 480Mbps.

I have smart TV that can play direct with external USB HDD. Thumb drives are not the same among them. The read/write speed can be slow and can be sluggish to stream video. Your old media player can support 4K video? If you ask my opinion, I'd prefer to buy mini PC like NUC because it is more flexible. The processor in TV is slow normally and once in a while the firmware is upgraded to add more feature and this will have more toll on processing power. You normally do not want to upgrade TV every 2~3 years

Cheap media player also tend to be sluggish and like I mentioned before the quality of picture depends on the media player hardware. That is why I would go straight to good mini PC. It can serve you for many years.