Bioload depends on how much rock you have to handle it. If you only have 75 lbs of rock in the 180 gal tank, your biofiltration is not going to be able to keep up.
Yes, you want to raise your calc and alk if your planning on keeping coral. you want your calc around 420 and alk around 9-11dkh I would check your saltmix to see what the calc & alk levels are from fresh made water.
If I understand correctly, you had an established FOWLR tank and moved it into the larger tank and are planning to do reef.
Your bioload may be some what stressed from the larger water volume causing the higher nitrates.
As far as your storage water, your saying your getting ammonia readings from it.
It is possible that there was detritus or something in the pump that you used that contaminated your fresh saltwater mix.
I can tell you what I do for storage of my fresh saltwater. I also use a dark colored 32 gal brute with lid. I do not seal the lid however. It is always open just a bit. I have a pump with hose in the container that is set on a timer that circulates the water twice a day for about an hour and a half each time. I also have a heater in the container that keeps the water at and a tem approx to the temp of my tanks. Give or take a degree.
I also use RO/DI water and it is check with a TDS meter.
Also, it is my understanding that all saltwater mixes will have a trace of ammonia. Your biofiltration will take care of it if it is well established.
I had a higher end salt mix registered an extreemly high ammonia level, between .5 & 1.0 ppm, freshly mixed and I ended up tossing the entire batch. About 100$ worth of salt.
As far as your nitrate levels, you should have a trace of nitrates. Average 2-5 ppm is not unusual. 20 + is getting to high, I would do a water change. Test your fresh made saltwater for all parameters too.