Do it, do it, do it! Schedule a trip!
Yes, off-peak is pretty much the same at DLR as WDW. As long as you avoid school vacations, the crowds shouldn’t be too bad. Just make sure you avoid the third week in February. I don’t know if President’s Week is a nationwide school vacation, but DLR is nuts that week. I think the entire state of Washington heads down there.
March is a great time to go as the crowds will be low. It can get chilly in the mornings and evenings though. And of course off season means not all the shows are running, but I’m sure it’s the same at WDW. As long as you get some weekend days in you should get a chance to see Fantasmic and whatever parade and fireworks show they have running at that time. I’m trying February this year for the first time…so we’ll see how the weather goes! I’ll be taking ingamba on his first trip, so hopefully the weather is nice!
May and the beginning of June (avoiding Memorial Day) are actually my favorite time of year to go. It is generally warm enough for me to be in shorts all day long, but the crowds are still low.