I'm not sure how to specify a type that is a list of another defclass-type. I found some reference code which had:

(defclass custom-type ()
  ...)

(defclass foo ()
  ...
  (bar
  :initform nil
  :initarg :bar
  :type list
  :custom (repeat (object :objecttype custom-type))
  ...)

However, Emacs didn't throw any errors when I set the value to a list of non-custom-types.

I found the use of -distinct to be more legible than the alternative. I'm open to suggestions as this uses dash in contrast to the existing implementation.

I've been thinking about this, my original motivation was to avoid adding an dependency
but if you'd like to use dash.el I'm ok with that.

Another question is whether we want to use the anaphoric functions, too.
Initially that would make it harder to understand the code but the same argument can be made
against the use of cl-loop. It should be fine if we note it somewhere in the documentation.

Another question is whether we want to use the anaphoric functions, too.

I don't think there is a anaphoric counterpart for -distinct/-uniq.

[...] the same argument can be made against the use of cl-loop.

Would you like me to swap the cl-loop out for a --filter?

Please don't take what I wrote as a critique of your code.

The point I'm trying to make is that the increase in expressiveness of any library or complex macro (cl-loop, anaphoric dash.el) comes at the cost of an initial hurdle to understanding the code for new developers.

That's ok as long as there's kind of “before jumping into the code, check out these docs” disclaimer,
and as long as we're in agreement on which (all?) features we'll use.

I'm fine with adding dash and using all the features it provides,
e.g. avoiding “wrapper-lambdas” when possible.

Not sure about how existing code should be handled,
refactoring it just to fit the newer style seems like a lot of effort
but we can just fix it as we come across it.