Allow Formulas to return a collection of just a specific field from Filter results

I have a Collection of “Roles”, where each Role is related to a User.

I want to create a Formula field that returns the collection of all the Users linked to the Roles in the collection.

But it’s not currently possible to return a collection of a particular field from the Filter results:

Currently we can only do this kind of operation for a single result, not for all items in a collection:

IIUC this can currently only be done by using a pair of Rules (Role Linked and Role Unlinked).
Which is better than nothing, but “Rule proliferation” is a real issue.

You can do it with two fields:
The first is a formula
and then the second is a lookup based on that
Filtered Roles > Users