Now that we have a E-V13 of obvious European (Paleobalkanic) origin in ancient Central Asian Steppes we can speculate on similarities of the name of Dacians and Iranian Dahae and off course the Diau / Tao in ancient Armenia.
What is the direction? From Iranian to Balkanian or the inverse? Or maybe chance similarity? Or common origin?
Furthermore I think the transformation of Daoi into Dakoi can be simply explained by the -k' plural suffix present in Armenian and possibly in some Balkanic languages.
According to Strabo's Geographica, the original name of the Dacians was Δάοι "Daoi". The name Daoi (one of the ancient Geto-Dacian tribes) was certainly adopted by foreign observers to designate all the inhabitants of the countries north of Danube that had not yet been conquered by Greece or Rome.
The ethnographic name Daci is found under various forms within ancient sources. Greeks used the forms Δάκοι "Dakoi" (Strabo, Dio Cassius, and Dioscorides) and Δάοι "Daoi" (singular Daos).[a] The form Δάοι "Daoi" was frequently used according to Stephan of Byzantium.
There are similarities between the ethnonyms of the Dacians and those of Dahae (Greek Δάσαι Δάοι, Δάαι, Δαι, Δάσαι Dáoi, Dáai, Dai, Dasai; Latin Dahae, Daci), an Indo-European people located east of the Caspian Sea, until the 1st millennium BC. Scholars have suggested that there were links between the two peoples since ancient times. The historian David Gordon White has, moreover, stated that the "Dacians ... appear to be related to the Dahae". (Likewise White and other scholars also believe that the names Dacii and Dahae may also have a shared etymology – see the section following for further details.)
Opinions on the origins of the name Daci are divided. Some scholars consider it to originate in the Indo-European *dha-k-, with the stem *dhe- "to put, to place", while others think that the name Daci originates in *daca – "knife, dagger" or in a word similar to dáos, meaning "wolf" in the related language of the Phrygians.
One hypothesis is that the name Getae originates in the Indo-European *guet- 'to utter, to talk'. Another hypothesis is that "Getae" and "Daci" are Iranian names of two Iranian-speaking Scythian groups that had been assimilated into the larger Thracian-speaking population of the later "Dacia".