I'm in a similar situation (aiming to beat higher difficulty stages, anything over 500k at the moment) and for the time being I only have six rainbows to work with. I've been running a page that has June Bride Lotus, Kugaisou, Colchicum, RG Black Locust/Robinia and Ghost Weed as the lead squad, but past that I only have Mei (who isn't bloomed yet) and a bunch of bloomed/evolved 5* gacha Knights.

That lead squad gives me +70% crit chance, +100% crit damage and 1.9x skill proc rate (Robinia's at skill level 2 so she adds 1.3x) along with a mish-mash of other useful bonuses (+attack power in general, +damage dealt to bosses, +attack power against bosses, bonus damage with solar bursts, Ghost Weed and Kugaisou combined guarantee that all Knights in the same squad as them will deal bonus damage to pierce pests, small heals and up to +30% attack power depending on how many pest nests the squad busts).

I only have 280 RG crystals at the moment since I upgraded Robinia to a 6* so I won't be able to upgrade someone else any time soon. Maybe by the time I have enough crystals Rainbow Rose's event will be a permanent reissue.