Feature summary:

MediaWiki:Delete_redirect_and_move text says:

The destination page "[[:$1]]" already exists as a redirect.
Do you want to delete it to make way for the move?

This should specify the target of the redirect, which can be pushed as $2:

The destination page "[[:$1]]" already exists as a redirect to "[[:$2]]."
Do you want to delete it to make way for the move?

In order to do this, you're first going to have to pull the redirect target with (I think) RedirectLookup::getRedirectTarget() and then push that to the system message as $2.

Benefits:

The system message would be more informative about the redirect page's content, giving users more information that would allow them to make a better-informed decision as to whether to proceed.

This is an initial step to implementing T378456: Add custom message (similar to delete_redirect_and_move_text) when moving page over a redirect that points back to itself, but probably should be done first as (1) it's simpler (2) has a more general application and (3) the output would be useful for fixing T378456, which it might not actually need to be if this is sufficient.