Use Grand Prix Super Series classification (https://en.wikipedia.org/wiki/Grand_Prix_Super_Series) - downside is that group of top-tier tournaments below Grand Slams is of very different size per season (from 2 to 30)
Use 9 tournaments per season with highest participation of top players - issue is with seasons 1970-1973 that do not have official rankings so participation needs to be estimated
There are several options: