Edit the next day:
I tried your idea with a slight change that made it even easier to switch over. It worked perfectly. I did not need to disconnect the network or do a factory reset and HDMI 1, in this case, supported the Express 4K as it should.
Beforehand, I verified the codes on an existing Roku Express 4K Do Not control the Roku screen on the TCL Roku TV. This test remote had no effect on anything on the TCL Roku TV. A few years ago I had just the opposite experience when trying the same thing using a new Roku Stick. Maybe I did something wrong back then.
1) I first configured a Roku Express 4K on another TV so the change over would be easier overall. It was a nice surprise to see that Roku made apps signing into a new device about 1000% easier. Not as tedious as I expected
2) I plugged the Express 4K into the TCL Roku TV and pressing any button the Express 4K remote changed the TCL Roku TV to HDMI 1. There was no bleed over from the Express 4K to the TCL Roku TV. Or vice versa. The TCLRoku Remote only works on the TCL side. The Express 4K remote only affects the Express 4K.
3) I need 2 remotes for family purposes. So I needed to reconfigure a 2nd remote to the new Express 4K. Only Roku remotes with the power and volume buttons work reliably. Some 1-click after market remotes prefer TCL Roku TVs, but will work on other TVs with a Roku player. I took a genuine remote from another room and configured it for the TCL TV / Express 4K and put the 1-click TCL remote in the other room and configured it for that TV.
Done. No need to factory reset anything. Everything works perfectly. The Express 4K is set to 4K picture / No HDR. Life is good again.
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Original reply:
That all sounds reasonable and I plan to give it a try. If it works I can put off buying a replacement TV for a while.
I looked up TVs with 1000+ nits sustained and there are very few and only about 5 rise above all the others as highly recommended by sites that review TVs. Perhaps another 5 - 10 sets are bright enough to support most HDR streaming videos and they cost less but each has gotchas mentioned in reviews that would detract from one being a main living room TV set. In general all are QLED as OLED does not get bright enough for everything artsy directors consider to be their masterpieces. Very few sets today, overall, come anywhere near 1000 nits of brightness sustained or otherwise.
Good HDR reproduction is supposed to require 1000+ nits of brightness, although not all HDR video is the same. Some shows are filmed so that lower powered sets will provide a decent picture. Amazon Prime is the worst for me as some HDR shows there look like they were filmed in a dark closet. HULU does a decent but not perfect job with HDR for lower powered sets.
And, I actually see HDR as a difference only and certainly not better in appearance. Old Fashioned SDR always looks fine to me and any old TV will do a good job with it.
My other alternatives are to switch over to FireTV or GoogleTV. Both have cluttered screens. FireTV is unreliable in controlling HDR on/off. I don't have any experience with GoogleTV and am not interested in a new hobby of testing streaming players.