How can I make my Seele stronger?
I've found Seele to be the most fun character to play in the whole game ever since I managed to pull her a few months ago (wanted to get her since I started playing, but she never had a single re-run since), and have been hard at work building her. I think I got her to a good place, at E3/S2 (Planning to get E6 and S5, am actively crafting additional LC's, sadly can't do anything to get Eidolons faster besides just pulling on banners as normal). One thing I know I could do is get Cerydra, but I prefer using Sunday, and would like to keep him. The 20% crit damage helps, and he advances Dan Heng's Dragon which also increase my survivability and gives more frequent cleanses. I'm also not planning on ever playing sustainless anything. Outside of those two factors, I'd like to hear some of you guys' opinions on how strong she currently is, and what I can still do to make her stronger still, beyond just getting more Eidolons and LC's.
In particular, I would also greatly appreciate advice on tuning the speed of Sparkle and Sunday around her, since I completely and utterly suck at speed tuning. I'm not particularly good with numbers in general, truth be told.