I can add some recent experience from July 3 and 4 this year. I visited for two full days and was able to see and do every attraction, show, parade, walk-thru, and dining I wanted in both parks. Before the trip I was anxious that I might need to buy the ultimate pass or individual ones, but ended up not needing either. During my visit the ultimate pass was $170 a day! Glad I skipped it.
I did not stay on-site but arrived at the normal rope drop time. As mentioned by others, traditional touring advice applies: doing bigger ticket rides early or in the evening and doing less popular things mid-day saves time.
One advantage of traveling during a more peak time than other trips I’ve taken to Disneyland Paris was that every restaurant and snack stand was open and operating their posted hours. Disneyland Paris has a reputation for having inconsistent food operations and low quality food. I was pleasantly surprised to not experience this. If you eat lunch right when an establishment opens, it is very likely you will be the sole person dining there and get great service and a choice of seats (presumably since Europeans tend to eat later meals). I ended up eating lunch both days at Toad Hall since the experience of piping hot food in an empty dining room was so good the first day.
Lastly, many rides have a single rider line that can save some time. Unlike other Disney parks, the single rider availability and corresponding wait time is listed in the app along with the regular standby wait.
Let me know if you have any more specific questions.