According to my understanding, the thing you use the past perfect to describe should happen before the thing you use the past simple to describe. Therefore, I wonder if my use of the tenses is wrong. On the other hand, I think in the second sentence, had increased works better with by than increased.
40% of households owned a washing machine in 1920. This number then had increased to about 75% by 2019, with a small dip in 1980.