I'd be interested in seeing your preview dash version. Perhaps your updated further than I am and further preview updates allow this. My preview dash definitely allowed me to install the dev app and looked identical to yours, but upon launching dev mode I was greeted with the error I posted previously.
I'll re-enable preview and then try dev mode again. I have no games installed and Retrix is simple to configure. I won't lose too much to try.
Newest Retrix doesn't perform any better on the fall update, but I cannot confirm that Retrix has been rebuilt for new dev mode specs (or if it even needs to be rebuilt) nor can I find confirmation that the fall update actually enabled the dev mode specs that it proclaimed it would.