I think your lowest crowd times of the year will remain the lowest crowd times of the year with the caveat that this year they will be much busier than ever before as many have been holding off trips or will make extra trip than usual to experience the hype. Even after the initial hype dies down, I expect that many will want to return to experience as it sounds like Disney have outdone every other effort in history in terms of immersion & storytelling, so I don’t know if it will ever return to the (even almost always busy now) norm that we have.
As for crowd distribution, I think SWGE itself will be the busiest of everything but DL and DCA will be markedly busy too though perhaps for everything outside of SWGE you’ll still be able to manage a great trip with the use the arsenal of Liner tips & tricks (i.e. rope drop, a good plan and FastPasses/MaxPass). I am interested (& pretty optimistic about the results) to see how/if Project Stardust will be as impactful as they hoped when they set out to alleviate choke points & congestion throughout. I am at very least glad they invested time, resources & planning to the thought of the space in existence & how to better manage the crowd flow through it. It would be SO much worse if they hadn’t for example done anything about the stroller stranglehold up the middle of Adventureland.
Now for all that they did internally to manage their infrastructure, I am woefully pessimistic about the situation external to the parks. There is no way the parking & even surrounding road/travel systems are adequate for the flow they are about to receive. I hope the city of Anaheim & Disney are working to figure how they will address/manage the parking lot that all the streets of Anaheim are about to become because until the new parking garage is complete, because it’s gonna be nuts. I am already so glad I don’t typically drive from home to Disney & that whole situation is about to be horribly compounded. And then there’s security to get in which has seemed to get busier & more backed up with each trip we take with no hope of relief once SWGE opens. We have had several recent trips where it took 45 minutes to get through security and another 20 to get into the parks. Usually coming back from a midday break, so we are hesitant to break back at the hotel after a certain time (noon-1) bc we have found that we need to get back by 4pm so we’re not caught in the main rush of crowds coming in for dinner after work and leaving later means a very rushed, might as well not even breaked, kinda break.
Anywho, all this to say, I think it will be awesome & manageable once inside the gates, but getting there is going to be the real pickle & me personally, if I were crowd adverse, I would look at late spring 2020 (after Easter/spring breaks are done) to allow for almost a full years worth of school vacations to pass after SWGE’s opening & before the summer vacation rush gets into swing.
Jan/Feb 2020 are other times I have seen suggested which are also fine so long as you avoid end of Christmas festivities, the mid-Feb part where lots of kids have “ski week” off and also make note that DCA is starting to do a lot of things to make this season a draw for crowds (Lunar New Year, this year a new Valentine’s Day celebration in DL & all the extra food festival offerings as they can!), plus more refurbishments at this time of year (although this year I’m pretty sure they doubled down on anything & everything in hopes that next year they won’t have to close down as much to help manage crowd flow for SWGE that will still be there in some form).
And as for what I am doing for opening day or soon after: with no previous plans to stay on-site anytime in the near future and only just one small discussion between us about possibly going for opening day, DH & I booked a 1 night stay at DLH for May 31st 
Note this is a full 180 of where I stood the night before the announcement where I went to bed dreading the change that would come to FastPass & passholder benefits (I was SO certain that FP would become a premium pay-to-play service with the addition of SWGE attractions and that the blockouts for passholders meant NO access to SWGE without the high premiums of a preview party or upgrading to a super expensive pass).
I was so pleasantly surprised on both fronts that it gave me hope enough to fully allow myself the excitement for all the neat things it will include & bring for better to the parks. We still may well cancel the hotel as we really weren’t planning on being anywhere near that crowd, but if the reservation system that they putting in place for the soft opening, especially with designated times for on-site guests pans out, we’ll be cautiously excited to be a part of opening day history.