Dialing in the hydration storage takes a bit of time. I tried bottles under the seat and behind the head. For short rides up to 100km depending on the temperatures, I go with a 1L Nike bottle behind my head in a triathlon X-Lab Torpedo Kompact cage. Many here seems to agree that that location for hydration is the most aero. But for anything longer than 80km-100km, especially in hot temps, I have a 2.5L (85oz) Osprey Hydraulics LT bladder in a Deuter Road One backpack that I wear on my chest. I use a powdered drink mix and fill it with ice and water. There is a 3L Hydraulics LT bladder that will likely fit, but so far the 2.5L has been enough for several rides from 200km-246km in September.
It doesn't seem to be much less aero than the 1L bottle behind my head, but having 2.5L of ice cold hydration on your chest feels pretty incredible in the Summer heat. Accessing the bite down tube while riding simply requires me to slightly turn my head to the left and it is right there. I never have to let go of the bars, pull a bottle from behind my head, or sit up to wrangle a bottle out from under the seat.
The bag also has 2 pockets for wallet, keys, phone, bluetooth earphone recharging case, energy gels and more. Since 1 of the pockets is on the bottom, I could put the 10,000mAh recharger in it with a cable, and connect it the Wahoo Bolt on the bars to recharge while I am riding, or connect it to my phone in the top pocket to recharge it.
The back of the back has 2 foam cushions that I'll likely trim off at some point to allow the cold to seep through to my chest, but for now those are still on. A good thing about this is that I can simply walk into a convenient store with the bag on my chest and have everything I need without having to dig through the Moose bag for my valuables.