But then it wouldn't seem like a 'supergirl' movie, and instead would just be another JL. Which would be fine, but I would want Supergirl to be the central, main character of her first movie in the DCEU.
Now, I would love Supergirl from Krypton, but it would be a juggernaut of a movie, spanning from Gotham to the Fortress of Solitude, from Themyscira all the way to Apokolips, and with all their assorted inhabitants coming along for the ride.
Good for a Justice League movie, but that is a team format and them coming together, whereas Supergirl should just be about Kara.
I think it should mimic MoS rather than JL in scope. Heck, they could break Supergirl from Krypton into two movies, with the first being a Supergirl movie, only with a different primary plot added, and then the secondary plot concerning Darkseid could lead into the second half, that being a JL movie to finish out an expanded story line.