Balance:
Being stable under water is like our upright walk. As far as the physics are concerned there is nothing stable about it. Our brain has to learn how to constantly make very subtle corrections that make us appear motionless. This takes time.
One way you can accelerate this process is by laying on the bottom of a pool in the correct posture, slightly negatively buoyant. Belly on the floor, knees and shoulders slightly higher, lower legs close to vertical and fins above and behind you, head back, face forward and arms forward and off the floor. That's going to feel very awkward, even painful, at first but just suffer through it. Then, inhale deeply to move up from the ground and try to balance yourself without much flailing for as long as possible. Try to slowly move the fins further back or closer to your butt and the hands further forward or closer to your shoulders with bent elbows. If you can't find a balance point this way, you have to move some weight or buoyancy around. Heavier fins, moving weights, whatever it takes. Then rinse and repeat the balancing exercise.
Like learning how to ride a unicycle. Get in the normal "riding" position while hanging on to something stationary, then let go of that object and try to maintain balance. Once you can hover in place, moving forward, backwards, and sideways gracefully is easy.
In between the pool sessions go diving and try hard to stay in the correct posture while finning. After a few weeks you'll wonder what was so hard about it. As adults we don't remember how many times we ended up on our face or butt while learning to walk.
Moving tanks:
How do I move double tanks that weigh about 100lbs? On my back of course, using the backplate and harness as a carrier. You can carry the wing regulators, etc. in a separate trip if you do not want to move the whole, assembled kit.
Custom drysuit:
If you go with DUI, call Faith Ortins and ask who she recommends to take your measurements or when she will be in your area to do it herself. If you shell out that much money, you want the suit to fit perfectly. There are many re-sellers that have the DUI measuring instructions but only a few people know where to add or subtract fabric to get a great fit. This is more of an art than a science. Cannot help you with other manufacturers.
Tank and backplates:
Nothing to add to the excellent suggestions from other posters.