We already had some threads about this topic on this forum.
It seems that definitly the proto-Slavic ethnonym was Serboi/Serbi (according to those mentions) in the 2nd century ?!
Have you read the whole Wikipedia article that you posted? It’s fairly short.
The ultimate first proto-Slavic ethnonym is unknown. The first known mentions of Slavs were written as Veneti, Sclaveni and Antes. That’s already 3 ethnonyms. These 3 terms probably refer to 3 groups of Slavs. Some source says that Veneti are ancestors of Sclaveni and Antes. I’ve read some article about a theory that says that the term “Sclavene” means “from Vene(tia)”, but it’s just imagination of the author of the theory. Never mind, the whole thing is very problematic, because the term “Veneti” has been used to describe a lot of peoples all over Europe.
The term Vistula Veneti (or Baltic Veneti) has been used in modern times to distinguish the Veneti noted by Greek and Roman geographers along the Vistula and the Bay of Gdańsk from other tribes around them and other tribes of the same name elsewhere such as the Adriatic Veneti (about the same area of today’s Veneto), the Veneti of Armorica,
and the so-called Paphlagonian Veneti (of today’s northern Turkey
coast). They are one of the tribes that are suspected as the ancestors
of some or all of today’s Slavs.
When it comes to Serboi, it is believed that they (whoever they may have been) came from the east and assimilated into Slavs. That means that Serboi cannot be the proto-Slavic ethnonym.
We have only a small number of sources, verity of which cannot be verified. We are working with raw data (opinions, beliefs, nationalistic theories, etc.), which can be interpreted in many ways. That means it’s a waste of time unless you’re a scientist that’s seriously trying to find some new information about the topic. But it’s good to know at least something about this topic.