Hey!
Just to answer your first post bold question:
Is there some kind of guide one pitfalls, gotchas, best practices… maybe an unfinished blog article on the matter?
I’ve written a few months ago a series of articles about my own experience of migrating a Phoenix project to Ash.
Here is the link: Migration to Ash series | Blog of the (p)repair project
It covers with some details and complete code example the migration process from an Ecto Schema to an Ash Resource.
I wrote it for me, as a way to validate my comprehension of the process and differences between frameworks approaches. I also wrote it with the goal to share it with the community. Because I spent a lot of time during last spring (April and May 2024) on the Ash Discord, and I received many help from Ash developers and users 
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Finally I had to switch my life project and to find a new job to get revenue (my first job as a programmer, and unfortunately it’s not in Elixir but PHP and JavaScript, but now that I start to get used of these “new” languages, I’m switching back to my (p)repair project on my free time). Well I guess, that’s out of the current topic 
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Hope these articles can help new Ash adopters which would came from Phoenix !
