As I mentioned, it would come from the actual overall damage increase Voidform currently adds.
Meaning you’re total damage no longer would increase but instead you get your increased damage from your active casted abilities which makes it more meaningful and noticeable making your Flay, Sear and Blast in their Void states actually feel chunky as they once were long ago instead of feeling like they don’t do anything at all.
I would argue that you have MORE uptime and thus more resilient to CC since you can gain access to those Void empowered abilities more often just like how Demonic from Demon Hunters allows in addition to its Metamorphosis which is akin to Voidform as it’s primary cooldown.
So for example, outside using Voidform cooldown in of itself, if you tie using Tentacle Slam to grant like 3-5 seconds of Voidform and thus temporarily empower your core spells, you have a small window to get out some hard hitting spells in a mini burst. This of course can be in addition to Voidform cooldown as well just like how Demonic works with Metamorphosis.
The concept is already proven to work for Demon Hunters since it’s inception. The primary difference as you mentioned is between the ease of movement being a melee vs a caster but that problem still exists without my proposal as it always has, the difference is that you gain MORE uptime in smaller more often bursts which again is more akin to Legion/BFA Voidform where you didn’t have a cooldown and instead you weaved in and out. But the primary difference is that you are not punished for NOT using your spells thus depriving you in forcing you out of Voidform faster. Instead, you have a set duration all the time so you only need to worry about the static time you get in those short windows and work within that frame in proper positioning and lining up your teams CC if that is your biggest concern and you can also hold until you are ready to burst which you did not have that option in Legion/BFA as it automatically put you in Voidform even if you were not in a proper position or have CC ready to go.
The extension I am talking about is that if you have an ability that works like Demonic in that it temporarily puts you into Voidform, then you are not punished when using it while Voidform is currently active and so the temporary time it would have put you in Voidform would instead just increase the current duration of your Voidform… exactly how Demonic works which is not as much of a big deal you are making it out to be.
Which is currently still a problem that my proposal doesn’t make worse but infact make it better since you have more opportunities to get into it. Obviously the best route to take is make it so it no longer requires a hard cast to get into Voidform with Voidform itself. That is a separate talking point which should be addressed for sure, but that only really highlights the current problem which is not made worse with my proposal.
Regarding the rest, I can agree that some things are better, but I also think improvements can be made and thus my proposed suggestions as I think Voidform in its mechanics is quite hollow and arguably could just be removed and make Power Infusion be the primary cooldown and so I would rather make Voidform more standout as a cooldown in taking what is a proven reliable system in both short and long fights and make it work for Shadow while also essentially fixing the issues that the original Voidform came with and at the same time changing the actual flow of the spec in speeding it up with how Mind Flay becoming the current Mind Flay Insanity is smoother to obtain and use while also bringing back Mind Sear in an on demand AOE in short bursts to both reign in its power while also making it meaningful and finally, making Mind Blast actually meaningful again in having it do chunky damage as it once did again with this short window of powered up state in Void Blast.
Sure it would require some rework in shuffling some abilities around, but all the pieces of the foundation exist, it would just imo bring it up to current design gameplay standards and fully flush out Voidform in a fluid form that is both proven to work and reminiscent to how Shadow played in prior expansions and thus be familiar yet modern.