Late September gets hot hot hot in Anaheim (mostly dry heat but is humid compared to the dry Utah climate I am used to). Crowds are pretty great for both though, as long as your dates don't linger and start running into Fall or Spring breaks that start right after both those times you mention. Weather-wise end of Feb/March can be rainy but this last year was unusually extra rainy and there were more than a few times that I felt very bad for those there for a once or rare in a lifetime trip because of the severity of the rain. But that is not the norm of California winters.
Another thing to consider with deciding dates so far in advance based on seasonally low crowds, is that DLR may (& does) schedule events/run promotions that aim to (& are usually successful in) drawing in the local following. Over the past couple of years it's been Food & Wine Festival, Festival of the Holidays, AP Days, & the most recent SoCal resident ticket promotion. So just a word of caution to go when you want to go/what works for your schedule and not base dates based solely on low crowd levels, since it can get so easily tossed out the window with some of these events that are often announced only within 30, maybe 60 days out.
Lastly, about FPs, as of last week there were a few changes that moved DLR in a digital FP direction. It remains to be seen what of these changes are permanent & which are "glitches" (as of right now it's possible at park opening to pull a FP with a window that starts just 5 min later meaning it is possible to pull another FP as that very near window opens up). DLR has kept announcements about the MaxPass (the next version of DLR FPs systems) very vague but what we do know is they intend to keep DLR FPs open to reserve only for same day availability (not in advance like WDW changed to). Also what had been announced is that they intend to charge $10/person/day for the ability to book FPs from their app on your smartphone.
So bottom line with FPs: lots is changing, and since you have plenty of time before your dates, sit back & let everyone else test it all, just keep tuned for updates.
One other thing I also just thought of, your Feb/Mar 2019 dates are around the time DL is aiming to have Star Wars land opened by which could be super exciting but would totally change the crowd level game.
So overall, go when works best for your life schedule, keep your eyes open for DLR updates, and enjoy all the months you have ahead to watch the changes unfold to see how they affect your day to day plans.