One of the most glaring issues that I am experiencing when using these two talents is that they both favor similar build paths/playstyles. Both cooldowns devalue the use of periodic damage spells, and when you have talent choices like Mind Sear, Mind Flay: Insanity, and Screams of the Void, all of these talents feel bad to use when both cooldowns choices incentivize NOT using channeled spells.
Dark Ascension does this in a boring and obvious way by simply increasing the damage of non-periodic spells by a flat amount. Void Form does this by putting a lot of its power in the introduction of Void Bolt and additional Mind Blasts. Even if Mind Flay/Sear was tuned in a way to be the optimal choice of spell usage during Void Form, the inherent design of the cooldown seems counterintuitive to that.
I would suggest changing Dark Ascension to almost be Void Eruption’s exact opposite, where instead of incentivizing the use of many direct-damage dealing spells in a small window of time, it now increases the potency of your periodic damage spells and encourages the use of channeled abilities like Mind Flay and Mind Sear. For example:
1 min CD
1.5 sec channel
Draw upon the darkness of your foes, dealing (70% of Spell Power) Shadow Damage over 1.5 seconds to all enemies within 10 yds of your target, healing for 20% of that damage done and gaining Dark Ascension for 10 seconds.
During Dark Ascension, your periodic damage dealing spells are 25% more potent, generate up to 100% additional Insanity, and increase the damage of your next non-instant Mind Blast by 4% for every second spent channeling Mind Flay, Mind Sear, and Void Torrent. This bonus is lost upon leaving combat.
I think this change would really open up more talents in the tree by avoiding the feeling of anti-synergy due to cooldown choice. This new Dark Ascension could also introduce a variety of supplemental talents; like ‘During Dark Ascension, you are able to move while channeling Mind Flay, Mind Sear, and Void Torrent.’ or "During Dark Ascension, channeling a single Mind Sear for more than 3 seconds causes your target to erupt with dark energy, dealing ### Shadow Damage to nearby enemies. "