Trying to predict opening dates is a fools errand, even when Disney gives dates.
MMRR was announced as "2019" then Summer 2019 then Fall 2019 then 2020 then Spring 2020 then March 4th 2020, which is technically Winter.
SWGE was announced as 2019 and then Fall 2019 and then Late Fall 2019 and then it opened August 2019 and then ROTR opened in December 2019.
So I'm guessing Ratatouille will open when it's done. I agree with
@HuskieJohn that there's a good chance they announce the date when they open MMRR.
As to boarding groups, I think that it will not open with them unless the attraction's reliability is a problem. I am of the opinion that 1 of two things will happen when MMRR opens:
1) it will use FP, in which case I think it's pretty clear Ratatouille will too
2) it will use BGs (probably called something different, like train car numbers), and they will prove so comically unnecessary that Disney will scrap using them for future openings.
Disney's history is that when they start FPs for a ride, they just appear one morning with no prior announcement.