I just ordered this set after reading your articles and reviews, but according to the specs from other sources and the details from big box stores, this set does not in fact handle 120Hz@4k resolution. It appears to only do so @1080p and 1440p. 60Hz@4k, can you confirm? No HDMI 2.1 according to specs elsewhere.
I've owned this 55R635 for less than a month. It was fine for a week. Then audio sync issues started no matter what I watch on it. Then I started researching and Roku forums are full of complaints about it. There are 11 pages of complaints. There seems to be no fix for it. This has been going on with Roku/TCL TV's for well over a year.
I spent HOURS last night trying to adjust and correct this, both through my new Denon 650H receiver and through the TV and through Prime Video/Firestick settings. There is no fix for it. The only possible fix I've seen is to completely disable all Roku by resetting and not connecting it to the internet and just using your own devices (Firestick, Streaming Box, DVD etc.).
This is really unacceptable. Half the reason I bought it was for the "cool" factor of the Roku built in to this TV and the high ratings of the Roku app. I could buy any TV but this one had what I wanted. And now in less than a month I'm extremely unhappy with it. To get proper audio syncing I have to disable the entire Roku feature of this!!
Reconsider your choice on purchasing this TV. Neither TCL nor Roku has offered any explanation or fix for this issue and it's been happening to many customers since at least 2018.