To minimize the number of values rendered NULL due to aggregation thresholds, youcan:

Broadening any filtering in place on the query is the correct approach because it increases the number of records included in the aggregation. Aggregation thresholds, designed to protect user privacy, will render values NULL if the number of contributing records falls below a certain minimum. By broadening filters, you increase the likelihood that enough records meet the criteria to surpass the threshold, thus allowing the value to be rendered instead of NULL. Reducing the time window would likely decrease the number of records, making NULL values more probable. Looking at dimensions in combination rather than isolation could also reduce the number of records per unique combination, potentially leading to more NULLs.