You can struggle all you like with designing perennial garden borders for timing and sizing and color but the single best way to make that border look better is to get the size right.
The rule of thumb I’ve always worked from is that a perennial bed has to have 1 foot of width for every 3 feet of length.
This means a 30-foot long border is 10-feet wide.
The maximum width imho – is 10-12 feet.
After that, it becomes a visual problem in most gardens. This means a 60-foot long border still has beds that are only 10-12 feet in width.
Get the width right and you’ll be amazed at how much better your garden looks right away.