I don't know much about Red Guardian but Void and Omega need sig 200 to be good.
I’d first like to say omega isn’t the only counter to the ddhk boss in v6. I just did a completion run today and used ghost for that fight. I think it took 3 revives but only because I’m not used to the new specials of ddhk so I got hit by the sp1 a couple times. Second, omega doesn’t need suicides to be good. I used my 4* for a few v6 fights, I don’t run suicides, and he still did well. He does need high sig though so I’d try to take his sig ability up asap. So in regards to your rank up question, all of the options are good but I voted omega because I feel like he is slightly better than the other two and he’s really good for v6 if you’re going to focus on that next. Hope this is helpful.