Instead of drywall, rather use shutterply. It is in your garage, so why not. You can drive nails into is for extra hooks etc. So on the "garage" side you can hang tools and other things that would be too heavy to hang on a drywall.
Have the bottom end stand in a "U" shape channel. That you lift off the floor by at least 100mm. Actually start at the top, the shutterply board should be just to short to stand upright and reach the rafters. But if you lift it a bit, you can nail, stapplegun or screw it into the rafter. Makes designing how to fasten the top easier. That bit you lift it would create a bit of a gap at the bottom. Have short 19mm square tubing feet or stands support that "U" channel.
The advantage. You can wash out the fishroom, just take the hosepipe in there and sweep the floor. Make provision that everything is sort of lifted so you can wash that salt layer that will form over time just out. You will flood and this is an easy way to clean everything.
Ensure that all the powerpoints are above the tanks and equipment. No cables on the floor, nowhere. All Multiplugs lifted and mounted to the wall.
When you do your layout, make sure two people can get past another without pulling in stomachs. My fishroom is 2m wide and at a point the layout was like a galley with tanks down both sides. That left a passage of 800mm in the middle. That was just enough to irritate me after a couple of months. Especially if you got a bucket or what not in your hands and need to get past the other person also holding something.
Have a open worktop available. At least 1m wide. With enough shelving to prevent your worktop to become a storage space. Place to do bucket transfer etc. If you can get hold of an old scratched TL550, get it as a holding system. Even a 60cm Daro tank might work. But that old TL550 or TL450 are great as they got all you need to house fish for a month after you done the bucket transfer. You can also connect your RO drum up with extra ATU to top up the holding system.
Have proper lights in there. So you can see what you are doing. I got two T5 double tube units.
Having that flowbin in there would / could be a problem in future. If you change your mind one day and need to get it out, you might need to break down a drywall. Just keep that in mind. I'm using 3 of those 220L drums. One for RO, the other two are connected to my system. The one is filled with liverock, the other is just water. This last drum, I can shut a ball valve feeding it, isolating it from the system. At the bottom drain, another ball valve can drain this 220L drum. Actually I drain my holding tank and refill that with water first from this drum. The leftovers I can drain to outside. Then I fill up this drum again with RO. Add salt and let it mixed. When ready I just have to open the top ball valve again and it is part of my system. 220L water changed done. Without carrying a single drum. I do have a single heater and old Seio powerhead in that drum to get the water temp up and to help dissolving the salt before I open the ball valve. I just need to remember to plug them out when I drain the tank. OK, these drums do hold far less than a 1000L flowbin, but the advantage is that they are not that bulky.