On moving the tank itself, it is a risk and I do often take that risk but I do some things to cut the risk. One thing to keep in mind is how heavy wet sand or dirt can be and how much danger there is in not being ready for the weight! We can often just about lift it, so we go for two guys to do the job! NOT good as that leaves you no hands free to open the door, hold the door again the wind and all those little things that get real big if one guy makes one small slipup!
Biggest danger in moving tanks with stuff in them is twisting, so I add safety by tipping one end up, sliding something that won't twist underneath that also sticks out to give good handles to avoid those slips. A 2X 12 a bit longer than the tank often works well for me.
And having a wife, girlfriend, kids, etc. around just for opening and holding doors or other small unforeseen things can really help a lot!
On planning how long the fish need to stay in "storage" has to figure in how much the sub will stir while moving and refilling with water and that is almost certain to require keeping them out of the tank for overnight or more, so I set up to let them stay as long as it takes. One reason I like the boxes is that they hold the temp well as well as giving a pretty reasonable amount of space for water for everybody. A styro box with a plastic trashcan liner can make a good "tank" for several days if needed.