I would preferred a "many worlds" take on time travel.
Have Jack and Ashi defeat Aku in the present. Big celebration. Victory. Then a beat of "wait, but..." Oh no, now Jack has to go back and defeat Aku in the past. And all of his friends will disappear forever!
They all talk it over, give him the thumbs up, big emotional jump into the portal together...
...Jack emerges in the past, kicks Aku's ass.
Now Jack's ending can play out SIMULTANEOUSLY with, in a parallel universe, everybody chillin and rebuilding the world, post-Aku.
And if you still want that final note of melancholy, find a reason that Ashi has to stay in the future (like maybe she can't go through a time portal she created, who gives a shit, handwave it). Then you can get some of that good parallel universe imagery of Jack and Ashi looking at one another through a pond and shit.
I'm satisfied with the ending we got. Coulda been better, less rushed, but desu, after 16 years, a functional ending is good enough. I'm just glad the story finally got some closure.