Angels are very much part of the Qur'an. See for example

And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day has certainly gone far astray.Q 4:136

Surah 35 makes it clear that these are creations of Allah, not some other deities

[All] praise is [due] to Allah, Creator of the heavens and the earth, [who] made the angels messengers having wings, two or three or four. He increases in creation what He wills. Indeed, Allah is over all things competent.

Q 35:1

There's of course the explicit adoption of angels like Gabriel and Michael from the Old Testament into Islam, but that's not syncretic; Islam in its own terms is an extension and clarification of Old and New Testament prophets. To find what the question asks for, "traditional deities" that "might exist as angels" in Islam, we have to look a little further.

Secular scholars will identify suggestions of the survival of pre-Islamic religious traditions, notably Zoroastrian, in some of the angels. This will be rejected by pretty much any Muslim theologian, so far as I'm aware. So for example the Qur'an names two angels, Hārūt and Mārūt, more than a little mysteriously-- and note here the association with Babylon and magic; eg they are plainly pre-Islamic and supernatural:

And they followed [instead] what the devils had recited during the reign of Solomon. It was not Solomon who disbelieved, but the devils disbelieved, teaching people magic and that which was revealed to the two angels at Babylon, Harut and Marut.

Q 2:102

Several western scholars have identified Harut and Marut with the Zoroastrian spirits, Khurdād and Murdād. Reeves [2015] outlines the argument and sources in a useful footnote

The names “Hārūt” and “Mārūt” are most often explained by modern scholars as garbled reflexes of “Haurvatāt” and “Amərətāt,” Avestan entities who correspond to the later figures “Khurdād” and “Murdād” and who govern the material realms of waters and plant life respectively. See J. Duchesne-Guillemin, Religion of Ancient Iran (Bombay: Tata Press, 1973), 137–38; A. Bausani, Religion in Iran: From Zoroaster to Bahaʾullah (New York: Bibliotheca Persica Press, 2000), 116. The source of this suggested explanation appears to be Paul de Lagarde; see the references cited by M. Grünbaum, “Beiträge zur vergleichenden Mythologie aus der Hagada,” in idem, Gesammelte Aufsätze zur Sprach- und Sagenkunde, ed. F. Perles (Berlin: S. Calvary, 1901), 63 n. 5. Possible confirmation for this interreligious correlation is found in a Central Asian Manichaean lexical list (M 109 recto), where line 16 of this Middle Persian–Sogdian glossary seems to make the same identification. See W. B. Henning, Sogdica (London: Royal Asiatic Society, 1940), 16 (text) and 19 (commentary); P. J. de Menasce, “Une légende indo-iranienne dans l'angélologie judéo-musulmane: À propos de Hārūt et Mārūt,” Asiatische Studien 1 (1947): 17.

Muslims do _not_ recognize these angels as "deities", nor do they talk much about them much. There is a very real hostility toward any hint of pantheism and syncretism is tolerated only to a point.

At a practical level, syncretism is most often identified with Sufis, but I can't find a Sufi "theologian" making an explicit argument accepting it-- they tend to be more focussed on mysticism and ritual as a way to get close to god, and they're less picky about just what elements they're adopting in service of that goal; I can't find them talking about alien gods or angels in this way-- but there's a lot of Sufi literature, and someone who knows it better may be able to point to something.

The danger of "deifying" angels can be seen in Islam's view of the Yazidis, frequently characterized by Muslims as "mushrikin" or "polytheists" -- this is not at all a good thing to be called in Islam, then or now. Islam was quite sensitive in conquered lands to the possibility of "smuggling in" heterodoxies in the way that Catholicism incorporated pantheistic elements into the saints.

You mention syncretism in SE Asia, and the closest I can get to that is the Javanese story of the hero Aji Saka, who travels to Mecca and meets the angel Azazil . . . I don't know if there's an association with a Javanese deity though.

The more common route for syncretism are what we often call "saints" (usually referred to as wali, but going by various other names in the Muslim world). Particularly in previously Hindu lands, you'll find that rituals and beliefs which were previously part of Hindu religion get attached to particular saints; for example in Indonesia, where the Hindu King Sunan Bayat -- apparently a convert to Islam-- is the subject of veneration; you often find the gravesites of these figures as pilgrimage sites, and more than a few of the rituals there may preserve elements of earlier religions.

This syncretism is today the target of Salafist violence-- so for example the Lal Shahbaz Qalandar shrine in Pakistan, a site which venerates a 13th century Sufi with a famously ecumenical worldview, who is associated by Hindus with the Hindu deity Jhulelal was attacked by the Islamic State's Khorasan Group in 2017.

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Sources

Some Parascriptural Dimensions of the “Tale of Hārūt wa-Mārūt”

‘Angels’. Encyclopaedia of Islam, Third Edition

Une légende indo-iranienne dans l'angélologie judéo-musulmane : à propos de H[a]r[u]t et M[a]r[u]t

Islamisation in Java to c. 1930

The Religion of the Kurds

Benazir en odeur de sainteté: Naissance d'un lieu de culte au Pakistan

The Politics of Sufism: Redefining the Saints of Pakistan

L'ORIGINE ISMAÉLIENNE DU CULTE HINDOU DE RĀMDEO PĪR

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In the post-Islamic era, many Muslim philosophers were highly influenced by Greek mitology and philosophy in many ways. For example, Avicenna (Ibn Sina) in his short story of Hayy ibn Yaqdhan, which is considered the only "novel" of orient, considered Greek goddess Venus (Ez-Zuhre in Arabic) like "an angel". I believe "some" Islamic scholars, especially the ones influenced by Greek mitology seem to think Greek gods and goddesses as real existing beings, like angels, but then became "gods" in a degenerated belief.

On the other hand, Avicenna's idea on angels is more complicating. For him, the "angels" declared as keepers in the Quran (82:10) were human intellect and perception. In some part, especially Muslim thinkers who believed that Qur'an verses have deeper meanings, associated with angels and jinns as such abstract things.

Likely, the names Harut and Marut are not mentioned in the Qur'an itself. Later Muslim scholars, because of their belief that all of the religions and books come from same source, they were very tended to associate mysterious Quranic stories with folkloric stories.