There are a couple ways it can be easier to do water changes with a sump. First, when dumping new water in, you don't need to worry about blowing sand all over the display tank. Also, if it's not the exact same temperature, you won't end up shocking your livestock, as the colder water will mix with the tank water as the sump pushes it into the display. Second, sumps are USUALLY located beneath the display tank. Most tank stands I have seen elevate the stand the about chest height, maybe shoulder height for shorter people. By having a sump, you don't need to lift those buckets as high, or pour them in so slowly.
But, it doesn't really address the issue of moving buckets around the house, or mixing salt water in buckets. One thing you could do is set up a five or ten gallon tank on a rolling cart that can be hidden in the closet for mixing. Toss in a strong powerhead and heater. When ready to do a water change, roll the cart out, fill the tank, roll it back out of sight, add salt, turn on pump and heater to mix and bring it up to temp. The next day, roll it out and pump the water into the display tank using the powerhead and some clear plastic tubing. I know, I know. I think of elaborate solutions.
Ultimately, the chore of transporting and mixing water never really goes away.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef