If we are committed to the idea of less Demonic Cores, then this is the change I would like to see: remove core generation from imps and hounds entirety and merge Demoniac with Power Siphon, but make it give 2 cores per imp. That, and remove the duration on Demonic Cores.
This would give the player exactly 4 cores every 30 seconds (25 if talented) and remove any fear of over capping cores, allowing the player to save or use them as needed. If the goal is to turn Demon Bolt into “a moment”, then removing the duration on the cores would let you stockpile them until Siphon is about to be off cooldown again, which coincides with Tyrant, creating a burst of 8 Demon Bolts once per minute. That means you can spend four of them before tyrant to summon your cooldown demons rapidly and still have resources to take advantage of Infernal Engine from Diabolist or Shadow of Death in Soul Harvester.
In fights with high movement, not worrying about overcapping cores or cores falling off means you can be more deliberate with your Demon Bolt usage or save up 8 casts for long periods of movement as needed. Having complete control over your instant cast resources would help you make the most of what you got.
Still, dropping from where we are now to just 4 casts of Demonic Bolt per 30 seconds is a big change, but if we are going to do it, at least remove the randomness of the cores and the pressure to use them up.