I have confined this answer from the perspective of Valmiki Ramayana only, for there exist multiple Ramayanas in different languages, and even in Sanskrit. Several Puranas and even Mahabharata (another ithihasa) reference it.
No. Not every character follows their Dharma. That might be the videomaker's oversight of the subtlety of Dharma & Adharma.
We'll use the negation principle to answer your question.
In any case, Ravana transgresses the Dharma in several instances. Ravana is the epitome of Adharma in the Valmiki Ramayana.
1. The forceful abduction of goddess Sita.
In Chapter 49, Book 3 - Aranya Kanda of the Valmiki Ramayana, Ravana forcefully abducts the goddess Sita.
- My counter-question is, what Dharma is there in Ravana forcefully abducting a woman, against her wish?
Obviously none. The Shastras mostly shun such acts.
As Jatayu says in III.49.6,11,12
कथम् राजा स्थितो धर्मे पर दारान् परामृशेत् । रक्षणीया विशेषेण राज दारा
महाबलः ॥ ६ ॥
"How a king adhering to probity can lay his hands on the wives of
others? If it is a king's wife, oh, great-mighty Ravana, she is to be
पाप स्वभावः चपलः कथम् त्वम् रक्षसाम् वर । ऐश्वर्यम् अभिसंप्राप्तो
विमानम् इव दुष्कृती ॥ ११ ॥ काम स्वभावो यः सः असौ न शक्यः तम्
प्रमार्जितुम् । न हि दुष्ट आत्मनाम् आर्यम् आवसति आलये चिरम् ॥ १२ ॥
You by your nature are a devilish and mercurial personality though you
have come from a decent lineage, how you have become an outranking
demon among demons and how you could attain kingdom, which is to be
ruled righteously, like an evildoer attaining a heaven-bound aircraft.
He whose nature is such, that nature is perhaps impossible to efface,
a sermon does not dwell in the heart of an evil-minded one for a long
Clearly, Ravana is not only Adharmic but also, evil.
2. The harassment of Vedavati
Chapter 17, Book 7 - Uttarakanda of the Valmiki Ramayana Ravana harasses Vedavati (in the age of Satayuga), subsequent to which Vedavati is forced to immolate herself to preserve her chastity.
एवमुक्तस्तया तत्र वेदवत्या निशाचरः । मूर्धजेषु तदा कन्यां कराग्रेण
परामृशत् ॥ २७ ॥ ततो वेदवती क्रुद्धा केशान्हस्तेन सा ऽच्छिनत् ।
असिर्भूत्वा करस्तस्याः केशांश्छिन्नांस्तदा ऽकरोत् ॥ ७.१७.२८ ॥
At these words of Vedavati, that Ranger of the Night seized hold of
the hair of that young girl, whereupon Vedavati, in indignation, cut
off her hair with her hand which had been transformed into a sword.
There is no Dharma in harassing pious women observing penance.
Not 'everyone' in the Ramayana was adhering to their Dharma. Ravana perhaps is the foremost of those examples.
What the OP heard viz.
every character of the Ramayana upholds Dharma: including Dhasharatha, Kaikeyi, Bharata and even Ravana. is thus proven wrong because Ravana was not Dharmic.
Regarding your last question
Are there good scholarly books/ research papers etc. that explore this perspective? - I'm personally helpless in that regard.