Yes, you still need charms for summoning, for the cannon, Idk.
I think there was a misunderstanding there, Marnie meaning do you personally still need your charms for 99 sum - not do you still need to get charms to train sum in general. As for range i think that the fastest way without a cannon/chins is ranging bloodvelds, but i'm not really an expert on cmb as you can probably tell by my cmb level lol (not deliberately 113, just haven't got around to training slayer much yet). If you want 20m slayer exp just do that though. Could melee some tasks/take a break from it if you get bored of ranging.