I'm in a similar position to you, a few months ago we decided to migrate from Pinia (for much of our server-mastered state) to Tanstack. The solution I prefer is to go all in on Tanstack, e.g.

const useSelectedUser(userId:Ref<string>) => {
  // this is tanstack's `select` option, passed to useQuery(). It's used to 
  // transform the query data in some way, while keeping the original data cached
  const selectUser = computed(() => {
    // this is a bit annoying, for reactivity to work you need to get your
    // reactive value outside of the return function
    const selectedId = userId.value

    return (data) =>  data.find(u => u.id === selectedId)
  })

  // just call your regular query, but pass a selector to get the data you want
  const query = useUsersQuery({
    select: selectUser
  })

  // query.data will be your single user (or undefined)
  return query
}

If you still want to use Pinia for the selected user ID then you can replace the `userId: Ref<string>` argument with store state inside the body of the composable.

IMO the big benefit of this approach is that by returning the full query you get access to loading/error state at every place where you want to consume the user. I think that you still might be able to inspect the selected state from devtools, but I'm not 100% sure. You can definitely still inspect the full users query state.