As I have stated in previous postings, one should not rely too much on Wikipedia for valid, reliable and accurate historical information. Although Wikipedia does provide good information, it is largely unedited and is often subject to inaccuracies and untruths.
The ethno-racial origins of the Thracians remains somewhat mysterious. There are schools of thought which claim that the Thracians may have actually been of Hellenic ethnic origin and like other Northern Greeks in the Ancient world-(Pre- King Philip), the Greco-Thracians would have been largely perceived as an unrefined and primitive tribe who were both both culturally distant from the more sophisticated and refined Greco-Athenians & Ionians. And that the "Hellenization" process, was, in essence, a quasi-Athenianization of the more primitive Thracian Greek dialect and culture-(similar to King Philip's quasi-Athenianization of Macedonia).
However, there is a general consensus among historians and historical ethnographers that the Thracians were probably non-Greeks/("barbarians") and in all likelihood, may have been of Illyrian-(or Albanian descent). Keep in mind that Ancient Illyria, was not just confined to present-day Northern Albania or Kosovo, but had also included much of the former Yugoslav, as well as the pre-Slavic Bulgarian interior. For example, 50% of present-day Bulgaria is comprised of the Thracian region, though the region of Thrace stretches into the Northeast corner of Greece, as well as the Northwest corner of Turkey......even the ancient city of Byzantium-(Constantinople/Istanbul), is part of the greater and historic Thracian region.
With regard to Thracian assimilation, specifically, "Hellenization", it is unknown as to when exactly the Ancient Thracians began to adopt Hellenism. I can't recall any of the major Greek or Roman Historians writing extensively on Thrace or the Thracians. What may have happened with regard to the Hellenization of the Thracians is due to the historical fact that Greco-Athenians began to settle the Northern Greek coast around 2600 years ago. This included, many towns along the Thracian coast-(and may have also included Byzantium). If the Thracians were Southern Illyrians or of another unknown ethnic background, then there is the likely possibility of cultural intermixing between the Greeks of coastal Thrace and the indigenous Illyrians living in the Southern interior. If this is true, then the Thracians would have gradually abandoned their indigenous language and adopted the Greek language, as well as the Olympian religion.
(It should be noted that Spartacus, was Thracian; however, was Spartacus of ethnic Greek or Illyrian descent?.......that I cannot answer. The Scientific Philosopher Democritus came from the Thracian town of Abdera in Northeast Greece and it is well known that Democritus, though Thracian, was of ethnic Greek descent. The town of Abdera is located in the Southern region of contemporary Greek Thrace. It is either on or in close proximity to the coast, which, in ancient times, would have distinguished where the Greeks lived versus where the Illyrians lived).
As for the Romanizaton of the Illyrians, there is little historical or linguistic evidence that I am aware of which tells us of any widespread cultural campaign on the part of the Romans to forcibly and universally assimilate or Romanize/Latinize the Thracians. I am sure the Thracians, like nearly every part of the Roman Empire, communicated the state language of Latin, though at that time, they would have also also communicated in Greek-(which was also the Lingua Franca of much of the Roman Empire), as well as communicating in their indigenous language, perhaps Illyrian.
"The Life of Greece" by the English Historian Will Durant is an excellent-(though somewhat lengthy and pedantic) historical account of Ancient Greece and may offer a discussion on Thrace and their ethnic origins-(though admittedly it has been some years since I read and studied Durant's "The Life of Greece'). But even Durant's information is still somewhat limited and is brought together by a scarcity of sources from what I remember.
Overall, the ethnic and linguistic origins of the Thracians continues to remain somewhat mysterious.