To BonkedAgain, I have (almost) the identical issue, except that the problem is limited to 4K broadcasts on YTTV.
Everything else works perfectly, including non-4K programming on YTTV and 4K programming on YTTV that I view in non-4K, as well as everything else that's not YTTV at all, including 4K programming from other sources.
My broadband connection and wi-fi is very stable and way more than fast enough to support 4K programming.
I've rebooted/restarted everything. I tried the "turn off the audio 5.1" trick. I did everything recommended and more. The problem persists--buffering every few seconds, always, for both live broadcasts and DVR'd ones.
I too have a TCL Roku TV, model 75R615 - a very expensive TV.
I suspect there is a problem with YTTV that affects me (this TV) specifically.
How did you learn of the problem you found about affecting your TCL model?
Your workaround isn't really a true solution, and if you know this to be a YTTV issue, YTTV must know it too. What are their plans to fix this? Do you know? Do they have plans to fix this?