As Geemo mentioned, Mar 3-7, 2020 is the Natural Product Expo over that Anaheim Convention Center. It might not necessarily have a noticeable impact on the parks (although it is a huge convention), but it will probably mean higher hotel prices in the surrounding area.
Given that your schedule is flexible, I'd also probably shy away from the week after Presidents' Day (Feb 17) because some schools get a mid-winter break and might opt to tie it into the trip. The upside to that week, is there might be longer hours and more entertainment if DLR thinks that the crowds will be higher than "normal" for non-peak season.
Otherwise, as others have said, the later you get into March, the more likely you are to encounter Spring Break crowds. Spring Break "season" typically starts in mid-to-early March and runs through mid-April (spring break isn't a specific week - it's scattered throughout March and April depending on school district). That being said, Spring Break "season" typically means longer hours and more evening entertainment.
F&W has been starting in early March lately and running through April. They've also offered the Egg-Stravaganza in the weeks leading up to Easter. You're best chance at experiencing both of these is to aim for late March (Easter is April 12, 2020). They probably won't announce the Egg-Stravaganza until much closer to Easter, but F&W dates might come out sometime towards the end of this year.
Personally, I'd probably try for late March. Despite the crowds, I like F&W, the Egg-Stravaganza, longer operating hours, and more entertainment.
In terms of refurbishments, there really aren't that many consistent closures (other than to install or remove ride overlays). I'd hazard to guess that probably one of the water rides (Splash or Grizzly) will likely be down during the winter months, but that's really just a guess.