Žambok is Hungarian, his roots are from
Debrecen in Hungary, so he shouldn't have a close connection with the others. These k13 results I posted are of the Marković family. Djurovski and Šaranac are the Macedonian and South Serbian I mentioned. I assumed the last 3 are connected.
Hromček also isn't a Serb. He just said he's a catholic from Subotica, his family settled there around 1800, and the surname was originally Hromek. It's found among west Slavs and in Croatia.
Todor is only in the FTDNA Q project? He's more likely Todorov or Todorović, the "Todor" surname doesnt exist there.
Vojvodina Serbs mostly came from the wider South Serbian region between 1690 and 1740. There seems to have been a lot of identity switching between Serbs and Romanians in eastern Vojvodina, I found such people on gedmatch, so that's a real possibility. Other than that no. Serbs assimilating into Hungarians was much more likely.
I'm not talking about the recent times here. (after ww2), e.g. maybe this Hromček and Žambok consider themselves Serbs nowadays.
There were earlier waves of Serb setlement in Vojvodina, but i think they were all assimilated into Hungarians. I never heard of a Vojvodina Serb with roots there older than 1690. Maybe Varda will know more.