I'm not sure, but it looks like the following comment in your code is no longer true

# If using Hotwire, you cannot import from @rails/activestorage
# instead, use skypack or similar to pull in active storage.

I was getting compile error using skypack and moving back to import { DirectUpload } from "@rails/activestorage" solved all my problems. I only have to deal with partial forms with hotwire now ;-)

 ☒ I think that "MessageVerifier-InvalidSignature" is linked to the form secret token not being properly managed. Not to validation.

 ☒ One quick question. How would you update and existing image?

 ☒ thanks. 
I have a has_one_attached in my model and so far it creates a new image every time I want to update one. So I guess that I probably missed something.
I also watched episode 281, which is a great companion to this one.