Rentaki's Soul Charm has a ramp up which looks like this:
0s: 1505 agi
2s: 3010 agi
4s: 4515 agi
6s: 6020 agi
8s: 7525 agi
10s: 9030 agi
12s: 10,535 agi
14s: 12,040 agi
16s: 13,545 agi
18s: 15,050 agi
which averages out to 8277 agi per second over the course of the 20s, similar to Juju's proc. SrS is "effectively dynamic" in that it updates every time you CoS, ES is snapshot so you could game the buff by holding a LNL for the last 3s if the holding isn't horrendously long, but it's likely to be a DPS loss holding it in practice. I suppose in SV you could game this proc with BA, but it'll be quite a bit of attention, and part of the gaming will be not delaying these shot too much to get into the "best tier" of the trinket proc. The trinket I imagine would be super powerful for ferals, who could rip and 5 point rake int he last 2 seconds and have those things snapshotted for their full duration, which would be crazy. For us I think it'd be too much of a loss to delay too much. For BM this trinket is just whatever, you will have 3 KCs in the span of the proc, I don't know that delaying them would be worth it for the next tier.