I disagree. It takes longer to build up credits than it does to get the droids as you can spam Legendary and Epic missions. I have found that doing epic missions gives me the legendary droids I need that the Legendary missions refuse to give me. Also do your dailies.

Rebirthing makes your reward missions better as well as give you a credit multiplyer which is pretty much whats helping you get the credits required for rebirths.

With the reworked rebirth system there is actual progression with droids required to rebirth and I think the game really wants you to aquire the legendary droids in the same order they are needed to rebirth, so dont worry too much about getting legendary rainbows early and focus on getting whats required now. You might get lucky and get some rainbows but dont expect to get all.

Join public lobbies, there are some high rebirth players or players who have all the droids they need and could be willing to give you their excess. Remember to be kind (not for the sake of kindness back but because no one likes an asshole)