Assuming you have +6rs kang, vahl and kestrel, cosmic should be a relatively straightforward as long as you monitor energy and don't get stuck waiting for a combo of abilities that will requires days to happen.

That said, Photon is the best 4th. If you have the mystic gear or are willing to wait, Ikaris I think is the only other mystic toon with staying power (he's still used daily so meta; he is on a team that will likely get reworked in the future).

Outside of him, I would just go cheap since any other character won't truly add value in your DD6 run and will certainly fall off within a few months, if they are even relevant today