How to obtain User Field Properties in Lookup List via Rest Api

I have a List (Topics) which has a MultiLookupField to a List named (Speakers)

Speakers has a User Field (UserName).

What I'm trying to do is to read some of the User Fields Properties. Which works when I'm querying the List with the User Field directly:

_api/web/getbytitle('Speaker')/items(1)?$select=UserName/Id&$expand=UserName

However I'm not able to get the User Field through the Topics list:

These are my desperate attempts which are all not working:

_api/web/lists/getbytitle('Topics')/items(1)?$select=Speakers/ID,Speakers/UserName&$expand=Speakers

_api/web/lists/getbytitle('Topics')/items(1)?$select=Speakers/ID,Speakers/UserNameId&$expand=Speakers

_api/web/lists/getbytitle('Topics')/items(1)?$select=Speakers/ID,Speakers/UserName/Id&$expand=Speakers,Speakers/UserName/Id

Edit:

I'm also facing the same behaviour when I'm trying to select Boolean or Choice Fields in the Lookup List.

Providing a little Image which should show my situation:

so for clarification my Topics List has a MultiLookup Column Speakers which is bound to the List Speakers. The List Speakers has some other fields and a User Field.

And I'm trying to expand some of the User's Field Properties through Rest.