It seems to me that you think about the question that you have asked in an inappropriate manner, so therefore you're looking for some confirmation of the existence of bhakti in the Vedas.
As far as I can see from your question your assumption is that the Vedas actually are the Vedic Samhitas which are the oldest, and only later emerged the Upanishads and the Bhagavad Gita and other scriptures in which we see quite explicitly clarified and defined in detail the doctrine of bhakti. So you differentiate between the earlier Vedic age from the later post Vedic age.
These are all the views and attitudes that come from the scientific community. But I must emphasize that all these views and attitudes the tradition has never agreed with, nor it is so presented in the Hindu dharma scriptures. What the tradition and scriptures teach is something completely different than that. Here I will try to illustrate what is the traditional view of these matters.
First of all we should know it is said that the Vedas are eternal. There are many statements in the scriptures that talk about that. Here is an example from the Manu-smriti, chapter 12:
"The Veda is the eternal eye of the manes, gods, and men"
- "The eternal lore of the Veda upholds all created beings"
Here the translator used appropriate term "lore" meaning "accumulated knowledge or beliefs held by a group about a subject, especially when passed from generation to generation by oral tradition".
Here the term "Veda" refers to all Shruti texts, which means that it refers to the Upanishads also.
The passage from the Brihadaranyaka Upanishad 2.4.10 speaks about the origins of the scriptures:
"As clouds of smoke proceed by themselves out of a lighted fire
kindled with damp fuel, thus, verily, O Maitreyî, has been breathed
forth from this great Being what we have as Rig-veda, Yagur-veda,
Sama-veda, Atharvâṅgirasas, Itihâsa (legends), Purâna
(cosmogonies), Vidyâ (knowledge), the Upanishads, Slokas (verses),
Sûtras (prose rules), Anuvyâkhyânas (glosses), Vyâkhyânas
(commentaries). From him alone all these were breathed forth."
So from the above verses not only we learn that the Vedas are eternal, but that they came out of the Lord's breath. All these scriptures actually has no human authors, and even the gods in heaven are not their source, but it is the Supreme Lord who revealed vedic knowledge in all these scriptures to the gods and men.
Puranas are even mentioned in the Atharva Veda 11.7.24:
"Verses, and Songs, and magic hymns, Purāna, sacrificial text."
Even from the fact that the Puranas and Itihasas were mentioned in the above verses from the Brihadaranyaka Upanishad and Atharva Veda speaks of the eternal nature of these Puranas and Itihasas. Since Brihadaranyaka Upanishad and Atharva Veda are eternal Shruti texts per the Manu-smriti verses stated above, it follows that the Puranas and Itihasas scriptures mentioned in these Brihadaranyaka Upanishad and Atharva Veda verses are also eternal scriptures!
Thus, knowledge revealed in all these scriptures is actually eternal knowledge, and so is the knowledge of bhakti also eternal knowledge. That knowledge as its origin does not have humans nor gods, but the Lord. In the Upanishads and Puranas and Itihasas including the Bhagavad gita there are clear references to bhakti. Here I will not mention these examples on bhakti because I guess you heard about them. Even in the Rig Veda we can find verses that can be interpreted as references to bhakti.
This would be from the traditional point of view. Knowledge of bhakti were not created supposedly later, but it is eternal knowledge about the relationship between the soul and the Lord. It is the knowledge of Sanatana Dharma which means eternal religion of the relationship between the living being and the Lord.