In theory it would be easiest to fill the display and then when it's full, it will start to drain into the sump and fill THAT. (Well... Don't FILL the sump, you need to leave enough empty volume for your tank to drain to the level of the overflows in the event of a power failure!)
But if lifting the water (buckets?) high enough to fill the main tank is a problem, you could fill the sump, then let the return pump fill the display tank... You'll just have to keep adding water to the sump... Fast enough to keep the pump covered, but slow enough that you don't overflow the sump.
In fact, having the bucket elevated just barely above the sump would allow you to just use a hose and siphon from the bucket into the sump... No lifting/pouring required!)
In the future, you can probably handle all water changes and top-offs through the sump.