Welcome to FlyerTalk. Delta is not a transfer partner for Ultimate Rewards. If you want to fly Delta, you need to consider who their airline partners are who allow points to be moved in from Ultimate rewards. Korean is the only SkyTeam partner to the best of my knowledge. I've only used points on Korean to fly Korean, but hopefully someone else has done so. Another possibiity is Virgin Atlantic. You can move UR points to them. However, I think there are significant fuel surcharges when redeeming on them.

The simplest solution would be to do as already suggested and redeem on United. Points transfer near instantaneously so I'd first find availability before moving them.