I have had 0 buffering on my FireTV Cube’s connect via ethernet. It is pretty much known the Roku app is very problematic, so I would not fuss that much with things as could be in the app.
How about trying a non Roku device and see if it will work fine. Then you know more unless the problem is your setup somewhere.
I did test the Gen4 on the DecoM5 system at my home in Memphis. There was zero buffering on the DecoM5 system. Both the Express and Ultra has no issues at all. Streaming and Gen4 were tested. The only buffering happened on the Zhone system running 100Mbps, which appears to be the bottleneck.
What’s FireTV bandwidth?
I think @rkkeller was referring to using the same modem/router, but switching viewing devices (Roku/Fire/etc).
I think I remember you saying you didn’t want to introduce new technology to your Dad, but as rkkeller stated, this would help you decide where the cause is, leading to which purchases you needed to make.
I’ve been playing recordings on both Express+ and Express 4k+ (via a 2.4ghz connection) without much trouble. I know you need live TV to work, and I will playing around with that today on those Rokus.