This is a strange issue, but I tested it a couple times, and it happened both times: When downloading my big (~3.6GB) fully-released game that I just re-exported via Linux(CEF) instead of NW.js and re-uploaded, a problem is that the Steam Deck completely freezes right at the time that the pop-up says the game finished downloading from Steam. Very odd. I wait a bit, then hold power down to force shut down, turn it back on, and run my game. It crashes at startup, so I say in game settings to verify files, and it says it found ~350 files it will reacquire. It does so, and then the game works! Which is awesome!
So the last challenge is: Is there a way I can prevent the Steam Deck console freeze at the end of the game download? Assuming this will happen to everyone's Decks, they'll all have to repeat the steps I did, which will inconvenience them, and I'll feel I have no choice but to include the steps in a troubleshooting area at the end of the game store description, which seems unprofessional and may hurt sales. Note that I first uninstalled the game, before downloading it fresh, but the freeze still happens at the end. I assume this isn't just an issue with my particular Steam Deck, since it never happened with any other game download. Thanks
(Also a side-question: Can I safely also use the Windows and Mac exporters in C3 to replace the previous NW.js builds of my game, and be sure that they'll work? Since I have no way of testing at least the Mac build, and my game is already out on Steam, so I don't think Steam support would test my game on other systems like I assume they did when I first put it up. I read in these two pages...
construct.net/en/tutorials/exporting-windows-webview2-2685
construct.net/en/tutorials/exporting-macos-wkwebview-2718
...things about signatures or something to run games on Windows and especially Mac, but those pages didn't specify if all that effort also applies to Steam games. I didn't do any of that stuff before with NW.js, and Steam support never told me my game didn't run on Win or Mac, and I only tested it on Steam Deck myself.)