Disneyland Paris uses dynamic pricing based on popularity. If you look at the booking calendar (goes out through March at the moment) you can get a feel for when things are busy because they cost more - typically weekends, but also school holidays. When I was researching, Tuesday was indicated as the least busy weekday, but not in the middle of summer when I was there because everyone and their brother was on holiday.
When you are there, there is a Disneyland Paris app that gives queue times for rides, and I found those accurate. If you don't mind not sitting with your travel companions on ride, the single rider line is almost always much shorter. If you stay at a Disneyland hotel (multiple choices) you get in to the parks an hour before general admission and that is definitely a less crowded time.
There are many social media groups and Reddit threads on best/cheapest/hacks, you might take a look at those.