I think you should find more White Creatures to better
balance the mana especially seeing how you need White for any of your "
deal damage base on Toughness" effects. 1/7 for 3W isn't all that good considering
Wall of Junk (perhaps too much $$) is 0/7 for 2. Not sure what your toughness criteria is but if it's 4 on the backside instead of 5 you may open up a lot more possibilities for quick drops in both blue and white. I might also suggest looking for evasion abilities on your "blockers" so that when they turn around to attack they get through. Searching I do see four 0/4 W options.
I do see going blue for the
Surge Mare and Charix which have offensive potential even without
High Alert/Huatli.
Fae of Wishes can be interesting as a 1/4 flier for 1U plus the wish where you could put something is a sideboard to search for.
Merfolk Secretkeeper is 0/4 for U but doesn't have Defender to worry about and could throw out a mill bluff.
Sorry for being so all over the place. In looking at making a dirt cheap ($$) tapdown deck I've thought of doing something similar using the cheap white creatures that have more toughness than power.