Looks good.
As for the plumbing,
Keep the overflows sized as is. Increasing them will not do anything. For the manifold, what do you plan on running off of it? If only the DT then yes just increase out of the pump to 1" into a tee then 3/4" out of each side with ball valves to control flow. It would be smart to ensure that the tee is centered between the 2 returns to avoid one return getting more flow then the other.
As for the plumbing,
Keep the overflows sized as is. Increasing them will not do anything. For the manifold, what do you plan on running off of it? If only the DT then yes just increase out of the pump to 1" into a tee then 3/4" out of each side with ball valves to control flow. It would be smart to ensure that the tee is centered between the 2 returns to avoid one return getting more flow then the other.