All that I know is that "Deva"/"Devi" refers to individual Gods and Goddesses, but we also use the term "Ishvara" and "Bhagavan". Aren't these two terms just synonyms for "Deva"/Devi"?
Bhagavan and Ishvara are simply Brahman as viewed by Jiva. There is no difference between Bhagavan and Ishvara and both may be conceived as epithets for the Supreme.
The issue of Devatas is complicated. There are different types of Devatas.
There are three aspects ... The three main .. deities - the Trinity consisting of Brahma, Vishnu and Siva - along wth their consorts, form the first aspect. Here all the .. deities are considered to be different facets of God, the Supreme (Isvara). The minor deities like Ganesa and Kumara, form the second aspect. Though these deities also are sometime described as the facets of God the Supreme [Isvara], their position is usually inferior to that of the Trinity. They represent limited manifestations of the Supreme God. The Lokapalas (protectors of the world), also called Dikpalas (protectors of the cardinal directions) like Indra, Varuna, Agni and others, comprise the third aspect. They are actually offices of power in the cosmic scheme of creation and human beings who have acquired extraordinary religious merit necessary for getting these places, will occupy them in each cycle of creation. Then there are any number of village deities and demigods who can be regarded as very limited manifestations of the Supreme God or as forces of nature deified or as human beings who by virtue of some special merit and power are elevated to godhood in course of time, after their death.
Hindu Gods and Goddesses by Swami Harshananda
I present an analogy which will clarify what has been described above.
Do you know what I mean? Think of Brahman, Existence-Knowledge-Bliss Absolute, as a shoreless ocean. Through the cooling influence as it were, of the bhakta's love, the water has frozen at places into blocks of ice. In other words, God now and then assumes various forms for His lovers and reveals Himself to them as a Person. But with the rising of the sun of knowledge, the blocks of ice melt. Then one doesn't feel any more that God is a Person, nor does one see God's forms.
The Gospel of Sri Ramakrishna, Chapter 6, The Master with the Brahmo Devotees (I)
Sri Ramakrishna is using the analogy of an ocean to describe Brahman. Brahman when thought of as Sat Chit Ananda may be thought of as an ocean of existence itself, consciousness itself, love itself. In this ocean are floating iceberg like structures which may be thought of as frozen consciousnesses. Brahma, Vishnu and Siva and their consorts may be thought of as comprising the largest icebergs with maximum power. Ganesha and Kumara may be thought of as smaller icebergs with more restricted powers. Then comes the even less powerful deities like Indra and village deities. Finally those who follow Advaita Vedanta consider jivas also as simply small bubbles in this ocean of Brahman.
I would like to give a example,
गजाननं भूतगणादिसेवितं कपित्थजंबूफलसारभक्षितम् । उमासुतं शोकविनाशकारणं नमामि विघ्नेश्वरपादपङ्कजम् ॥
विघ्नेश्वर (Vighnēśvara): Vighna = problems and Īśvara= master or controller Thus then, Vighnēśvara = the master or controller of all obstacles. (Source)
From viniyog of Aditya Hridyam,
आदित्यह्रदयभूतो भगवान् ब्रह्मा देवता
Here, भगवान् (Bhagavaan)= principle or eternal God. देवता (Devta)= deity
So the complete meaning comes out as, "The deity (of aditya Hridyam) is the principal and eternal god within the Heart of Aditya".