I've been surfing nicely on my TCL 2018 Roku TV, nothing to buy/hookup, same TV remote.... until lately been trying to watch some 4K youtube video and BUFFERING! what is going on? surely my 100 mbit wired Internet should handle it, then I fire up youtube "stats for nerds" just to find out am maxing out @~30 mbit, WTF! then I re-run the same test with an old Roku 4, and now am going 50 mbit, still buffering, then re-run again with my ATV4K, NOW pushing 100 mbit and no buffering. This has to be a weak TV CPU/GPU not delivering the performance now demanded. I broke down and got me a latest 4802x Roku Ultra and everything is now right with the world, just a little tricky using two separate Roku remotes now.
I wonder if your TV was beginning to show it's age. I don't know what the average lifespan of a smart TV is before it begins to show signs that it is starting to die. But honestly, five years doesn't feel like enough time for a TV to start dying.
I've only owned a total of three Roku TVs over the last few years. The first was bought on Black Friday and had it's backlight fail two years after it was purchased. The second was bought around the middle of the year and I would still have if I hadn't accidently broke it's coax port while attaching an antenna to it.
My third one is only about a year old and is still running fine.
Oh TV is not dying and youtube helpfully picked the stream that wouldn't cause buffering but my expectation was that any 4K TV should handle 4K 60hz, WRONG. The buffering occurred when I PUSHED it. I can feel between the TV App and the Roku App interface is smoother with the external box, and the HDR effect more pronounced.
I have a 5 year old Sharp Roku TV. My TV never buffers when playing 4k content on YouTube. My TV is connected to my 300Mbs fiber optic internet from Verizon, wirelessly.