Info from the gem reveal changes a lot of things

Updated PoB: https://pastebin.com/9GRpgn6g .

Black Cane vs Prism Guardian:

If you want to go the Black Cane route, I calculate it to be about 1.47 mill dps vs the 1.63 mill of Prism Guardian version. Don't take the dps numbers too seriously as this was rough estimation in an excel. This doesn't include the phantasm DPS, which is about 385k dps assuming 0 frenzy charges (realistically they will get charges).

In theory yes it's more DPS with the same eHP, though you pay for it with quality of life and the risk of phantasms dying. Furthermore, Prism Guardian will be cheaper because you can go lighter on the ele res on the rest of your gear, or get dps affixes you wouldn't be able to get otherwise. I also didn't include any jewels - hypnotic eye jewels with added phys to spells will give you a shitton of dps on Prism Guardian version, but will have diminished value on Black Cane version.

On the topic of Black Cane boosting Cruelty DPS, losing a link to Phantasms as well as losing Pride makes it have much less of an impact. The difference is 44% more DoT on Cruelty on Sirus for Black Cane vs 42% on Prism Guardian (Using Sirus HP and already including the 25% inc. effect on the support gem). You can scale it a lot higher if you put your phantasms on a different link, with like cwdt + ball lightning + culling, but then you lose all your phantasm dps.

In conclusion, go whichever way you prefer. It's trivial to switch between the two, just a couple regrets, gem swapping, and changing one piece of gear. I'll add the final Black Cane version below once I finish tweaking the auras to match the mana reservations.