@iodine @Rewilley @mstnggt66 @RokuKariza-D
The good news: I was finally able to resolve this issue with absolutely zero help from This Old House Ventures. After multiple times trying to get them to respond, I gave up on them. Dreadful customer service.
Then I decided to try roku support again. I started with online support chat, which is an AI chatbot and is nearly worthless. I then spent two hours on the phone with Roku support (a feat not easily achieved; they don't make it easy to find the number (816-272-8106, you're welcome), and if you select the wrong option they say they only support billing questions and new installs and promptly hang up on you). I finally got connected to a tech support guy who was surprisingly diligent in troubleshooting the problem.
The key clue came when I decided to log into guest mode and install the ToH app there...and voila, it worked. But after logging out of guest mode, the same problem was occurring (it never progressed beyond the splash/logo screen). This confirmed for both the support rep and myself that the problem is definitely tied to the Roku account somehow.
The bad news: you're not going to like the answer. The resolution was to deactivate my Roku account, canceling any subscriptions, and recreating it from scratch. Then a factory reset, logged into the newly created account. After that, the ToH app loaded as normal, finally. I had to log into all of my services again and repeat the process for all of my other roku devices since they were also logged into the same deactivated account.
If you're waiting on ToH to resolve this, you will probably be waiting for a while. If you don't mind creating a new account, this path works. Hope this helps!