the damage needs to be competitive.
You’re missing the point. The damage cannot be balanced to be competitive given the design drawbacks the spec currently is based upon.
From its very most basic idea, to the clear intent of the developers, the design for Voidform has involved very long ramps, and positive feedback loops.
The way the model is intended to function is that Shadow has no burst cooldowns, and multiple long ramping mechanics that take minutes for it to get going, but when it does get to full power it should do more damage than other specs who do not have those drawbacks.
The outcome should be that Shadow deals less damage than all other specs, on all fights which are shorter than the time it takes Shadow to fully ramp up, and then average out the ramp time. This describes ~95% of encounters in this game which are not raid bosses. This has always been true in Legion, BFA, and Shadowlands.
But, once fully ramped up - the upside of those long ramps should be that Shadow does more damage than every other spec in the game under those specific conditions: enough uninterrupted combat duration to fully ramp and average out.
That was the case in Emerald Nightmare when Voidform was originally implemented. Then it was nerfed, obviously, because it was unfair for Shadow to be the best raid boss DPS spec for all bosses - despite all the weaknesses borne in all other content.
Since they are unwilling to execute on the boon of this design (superlative boss DPS), they must remove the penalties associated with it: all the long ramps. That includes Mass Hysteria, Chorus of Insanity, Voidform Haste, Lingering Insanity, Shadowy Apparitions, Insanity decay acceleration, etc.
All of that must go.
They could keep Voidform the aesthetic spell. They could even keep the 2.5 button rotation - which I think is what you like about the spec currently (though personally I wish they wouldn’t).
But that’s a full redesign of the spec, while keeping the mechanic names the same. Asking for Shadow Orbs back seems like less of a burden on them, than asking them to give us something entirely fresh.