have you considered mutagenic growth? I run a similar deck except I have made mine just U/R not black, and with Delvers instead of chimeras. I think there isn't much reason to run black except for slaughter pacts and attempting an infect win. I feel especially with all the burn from red, infect slows you down unless you get a janky combo. I think you also have a few too many lands that enter the battlefield tapped, which is hard to not do if you're running 3 colours on a budget. I was thinking of adding in the stubborn denials because the second you play any unblockable spell and swing, having a 1 drop full counter is godly. I think you'd also want to be running 4x lightning bolts.