Why use harmony and not the word balance. Actually the word doesn’t matter, because you always give your own interpretation and definition to a word. Sometimes that happens very intuitively or because that’s how you learned the word.
For me, however, there is a difference. Balance makes me think of how life should always be balanced. Equal amounts of rest, equal amounts of busyness, equal amounts of time spent working or relaxing. There almost “should” be a balance in this day, in this week. Like being on a scale that otherwise drops to one side. And that doesn’t feel appropriate for what I’m doing.
Perhaps not strange if you look at the definition of the word balance:
ba-lance (the; f(m); plural: balances)
1 two-armed balance: to tip the balance the matter decide
2 balance: to be in balance in equilibrium; to be out of balance
3 (bookkeeping) tabular overview of the values of a company’s assets, liabilities and equity on a given date: draw up the balance sheet calculate the result of his work (also figuratively)
source: Van Dale

Sometimes life asks a little more of you than you would like. It is certainly important to recognize and see that, so you can decide what to do with it. Sometimes there is no other way. Then it is nice to give a little more time to silence and rest after such a period, so you can recover.
Harmony is not only in life, but I think of art and creativity.
For example, mixing paint where I get the most beautiful colors when I mix pigments without using the same amounts. Or looking at a painting where there are not the same number of lines or the same colors used. Notice then also that your eyes keep moving and you want to look at that painting even longer. Harmony, not necessarily balance, makes art special … soothing … exciting … innovative … stimulating …. etc…
It also sometimes feels a bit like music.
In a very energetic piece of music or song, a quiet violin or piano moment can actually sound wonderful and stand out, giving time to breathe, to then continue on energetically. Or the seasons we have in nature, within which there is not always a perfect balance. But if we look at the whole of the different seasons, there is (normally) a beautiful harmony between spring, summer, autumn and winter which allows nature to keep itself alive very nicely.
Look also at the definition of this word:
har-mo-ny (the; f; plural: harmonies)
1 agreement: living in harmony in good terms, without conflict
2 (music) pleasant-sounding union of tones
3 music ensemble playing wind and percussion instruments
source: Van Dale
Beautiful here are the words “in tune” and “union of tones.” It is tuning in each time to what life is asking of you at that moment, what you can give and what you need. Because it can be hard to balance everything in your life perfectly. But realizing what is there now and acknowledging what you need gives more peace and beautiful harmony in life.