I do not consider myself an expert on Thracians by any means, and I do not feel qualified at all to comment on the Dacians, but I can offer a partial answer based on my current PhD work and knowledge of Greek history before the arrival of the Romans; hopefully, someone else can clarify or address my shortcomings here. :)

Broadly speaking, the Thracians comprised a large number of peoples or ethne (often labeled in scholarship as "tribes") originally inhabiting the geographical area known as Thrace, the extent of which remains rather ambiguous, though ancient sources sometimes identify the Strymon River as its western boundary.^1 Greek sources also state that certain Thracians occupied or migrated to/from parts of Asia Minor, particularly Phrygia; linguistic and material evidence may lend some support to these claims.^2 Other and probably less reliable traditions place Thracians further south in places like Boiotia and Thessaly, attributing to them the foundations of a few poleis and also associating them with the Pelasgians, the supposed aborigines of the Greek mainland.^3

These peoples shared cultural, religious, and social practices, and while it remains an open question whether the average individual would have initially thought of him- or herself as a Thracian, epigraphic evidence for Thracian self-identification already begins to appear around the mid-fourth century B.C.E.^4 There also existed a Thracian language, but we know very little about it, and the surviving inscriptions all employ Greek letters; unfortunately, research on the Thracian linguistics has often suffered from overly speculative and/or nationalistic (i.e. on the part of Bulgarian and Romanian scholars) approaches.^5 Of course, Thracians also fought among themselves, yet constant warfare eventually brought many of them under the rule of the Odrysian Kingdom.

Classical authors frequently paint negative pictures of Thracians---motivated no doubt by the occasional outbreaks of violence between Greek settlers and the indigenous populations in Thrace.^6 Likewise, Thracians could embody the exotic and liminal "other" in Greek perception, taking on the many stereotypes (both positive and negative) regularly attributed to "barbarians."^7 Yet literary and epigraphic sources also allude to many instances of peaceful coexistence and exchange, with a number of poleis even characterized by their "mixed" populations (e.g. Dionysopolis) and by their preservation of Thracian toponyms and foundation stories (e.g. Mesembria).^8 This fits well with Zosia Archibald's recent argument that, within the east Balkan-north Aegean regions from the fifth to first centuries, "the idea of a cultural divide may have existed at an abstract, intellectual level, but it did not exist in spatial terms."^9

Given these complexities (which I have greatly simplified here), I would caution you against viewing Thracians as some monolithic group; and you can perhaps extend the same caution to the other peoples that you listed. Nor should you derive too much significance from their differences with Greeks (possibly an even broader and messier concept!), since these differences may not have constituted a major barrier in the day-to-day interactions among the populations of ancient Thrace. I mean not to underplay the shared customs and language among the Thracians but rather the impressions of "foreign," "exotic," "barbarian," etc. imposed upon them by the prevailing classical traditions and sometimes even modern scholarship.

Footnotes:

^1 See e.g. Pseudo-Skylax 66.5. On the issue of Thracian ethnicity and identity, see esp. Denver Graninger, "Ethnicity and Ethne," in A Companion to Ancient Thrace, ed. Julia Valeva et al. (Malden: Blackwell, 2015), 22-32.

^2 See e.g. Eleonora Petrova, "Bryges and Phrygians: Parallelism between the Balkans and Asia Minor through Archeological, Linguistic and Historical Evidence," in Thracians and Phrygians: Problems of Parallelism, ed. Numan Tuna (Ankara: Centre for Research and Assessment of the Historic Environment; Faculty of Architecture, Middle East Technical University Press, 1998), 45-54.

^3 See e.g. the polis Anthedon, according to Stephanos of Byzantion A 319.

^4 See the so-called "Pistiros Inscription"; for later evidence, see e.g. I.Kaunos 148a).

^5 For a good introduction to the topic, see Anna Panayotou, "Greek and Thracian," trans. Chris Markham, in A History of Ancient Greek: From the Beginnings to Late Antiquity, ed. A.-F. Christidis with Maria Arapopoulou and Maria Chriti (Cambridge: Cambridge University Press, 2007), 738-744.

^6 On this, see e.g. Joseph E. Skinner, The Invention of Greek Ethnography: From Homer to Herodotus (Oxford: Oxford University Press, 2012), 83-86.

^7 See e.g. Despoina Tsiafakis, "The Allure and Repulsion of Thracians in the Art of Classical Athens," in Not the Classical Ideal: Athens and the Construction of the Other in Greek Art, ed. Beth Cohen (Leiden: Brill, 2000), 364-389.

^8 On Dionysoplis, see Pseudo-Skymnos F 2b, ed. Didier Marcotte, Géographes grecs, vol. 1 (Paris: Les Belles Lettres, 2000). On Mesembria, see conveniently María Luisa del Barrio Vega, "The Greek Language in the Black Sea," in Studies in Ancient Greek Dialects: From Central Greece to the Black Sea, ed. Georgios K. Giannakis et al. (Berlin: Walter de Gruyter, 2018), 511-530 (521-523).

^9 Zosia Archibald, Ancient Economies of the Northern Aegean: Fifth to First Centuries BC (Oxford: Oxford University Press, 2013), 127.

EDIT: Replaced "eastern" with "western."