Sigilaria is turn and skill act based while Aeonium is not. I think poor Aeonium still have chances to shine. Not in every single battle but she won't be forever benched like some others.
I think it heavily depends on what you're facing and what is your team purpose. Debuff is good but kinda useless if enemies have buffs to counter that (like +100% attack on a high attack stat or +100% precision). As far as I know, debuff only affect raw stats and not stat after buff application. Buff are good if you have a good someone to recieve them. DPS works only if you can survive to deal damages and tankiness is useless if you don't have DPS behind.
I think there is not an ultimate team to win everything. I think engame is more about balancing and playing around bosses weaknesses (not type weakness but game style weakness).
And I think Bamboo excels at being meh.
Speaking of Knotweed valentine, you may want
Sedul Easter. She will be available on the 30FS easter gacha. She may worth her 150 gems.
edit: I just noticed that Sigilaria has the lowest ATK stat of all 6* with only 4407 max (without centies and ampule), folowed by Bamboo's 5036. It's even below the weakest 5* ATK (
Showy Baby's Breath with 4466) and only 3 points above the second best 2* (
Salvia with 4404 ATK).