That's how I see it in this sequel. Darkseid travels back in time to take out on overwhelm the younger, more vulnerable, the teen versions of Clark, Bruce, Diana, and the rest of the group, but he also wants to level earth .
It's basically the Terminator franchise concept of destroying the savior in the past so you can excel in the future, though, in this case, Darkseid has decided to bring a whole future army to kill the yet to be JLA members and to decimate the planet ,as opposed to just sending one guy as he did with Steppenwolf. I imagine him basically unleashing everything in terms of his fleet and his creatures .
It's a bitter revenge for being thwarted by the future JLA several times in his timeline, so ,I see it in his mind, as a long time coming type of revenge. Reverse Flash has a similar motivation in the CW Flash show, in that, he's been beaten so many times by the Future Flash, that he gets great satisfaction to defeat the younger and more inexperienced Flash.
As for arrow, I guess it's a bit like the Hawkeye issue in the Avengers films, though ,even more so in this case ,since he's basically a Green Arrow in training, and I see him being like 21 or 22 at this point.
I see young Bruce enlisting his help because he trusts him, and because The Flash has told him he's one of the future members of the Justice League .
Plus, they'll need all the help they can get. I see him basically fighting Parademons on the ground level on the chaotic city streets as opposed to taking on one of one of Darkseid's massive ships are his larger beasts.
It's gonna be an even greater challenge since in this story, the young heroes are each at different parts of the earth when the invasion happens, and they have to reconnect in the midst of this carnage.
It's also compounded by the fact that while Barry was able to warn the group about Steppenwolf in time to stop him, Barry's now stuck in the early 1990s, and doesn't know that Darkseid will bring a full army to the early 90s.
So, our young heroes, and the world are in real pickle, so to speak .