Good plan ! With a bit off luck you might meet Mapple training there somewhere
So the Gacha is 5K DMM points 11 pull with one Random 6* guaranteed, yes ?
You get one of the rainbow from the pull, the one that you don't have in the team.
If I can choose, then Kerria from 2015 and Loquat from 2016 gacha. But yeah, I'm not gonna try my rotten luck on random gacha ever.
FKG ID
DMM ID: 561075604 (106/107)
IGN: Dusk
[Just a guy with bad self control and blow his gems for more salt and sodium chloride]