Dendrobium will slowly fall, that normal. She handle 80% of the game by herself but is not good enough for the hardest 20%. Poor lady is too old now.

Your Dendrobium team is not agressive enough. Honeysuckle deserve her own team. I tried both of them too but it just don't work. It's better to have one main and 4 support.
Ash is not good too. She just bring Slash and Magic attribute and somehow -30% Def. You don't care about def and guts for turn 1 team anyway. I'd say she is not worth the slot.
Colchicum is OK. Ume too I guess. Her Crit buff is welcome.
Persian Speedwell will be a good choice.

Here is mine for Ex event map:

Btw, Spring Start and Cattleya are just School versions with alternative outfit. Kugaisou was here for the previous daily Ex event map to hit both Hit and Pierce.
Cattleya is nice but not optimal for turn 1 team though.