check prestashop metrics
https://addons.prestashop.com/pl/analizy-statystyki/49583-prestashop-metrics.html
or
https://addons.prestashop.com/pl/analizy-statystyki/27138-google-analytics.html
or something like this...
You want add one percentage promo for all product like -20% for all shop? In build-in options u can add X discount for products from only 1 category this is not enough?