This is 4 Forma in total because you do not need the V forma on my Venom Dose, this is just to show it works with my other build. While I have shown this build complete with a - Forma for [Fleeting Expertise], I have personally used a U Forma instead. This is because if you want to switch between my two Saryn builds, then you'll need both a - and a V forma for Exilus or instead a U forma for Umbra Intensify. Here is my Nourish/Xata's Whisper/Roar/Eclipse Saryn: https://overframe.gg/build/514318/
For this build to be complete: You need to use 3 Crimson Ability Duration as it will bring the cost of Gloom to the very minimum as well as buff the duration of both [Venom Dose] and Toxin Lash to a comfortable level. I suggest you use 1 Tau Duration and 2 regulars duration to achieve the 1.88 max channeling energy cost for Gloom because that's what I use. To achieve the 272% Ability Strenght required for Max Gloom Slow, first, you need to equip any Crimson Ability Strenght Shard, then kill 250 enemies to max Molt Augmented, then use either Molt [Vigor] or Madurai Chained Sling, and finally use [Growing Power]. If you use Madurai chained Sling then you don't need to equip one of Molt Vigor or Molt Augmented, or Growing Power, or vice-versa. Finally, 1 Amber Casting Speed to make the cast time of Gloom and other Sharyn's abilities more comfortable to use.
Disclaimer: I am not sure how good this build is without archon shard supplementation, I might remove mine to see how it is but I think it should still work fine. This build might not be the best in the world but so far it has worked well for me. In the future, I might revise this to be built with less Forma and rely less on Archon Shards. Also, [Streamline] works instead of [Boreal's Hatred], I was using it previously and wanted to use [Rolling Guard] or [Adaptation] but it increased my energy cost instead, then I found Boreal Hatred maintained it while also giving a bit more shields so I now use it instead. In hindsight now that I am looking at overframe it's because of my duration archon shards so if you use [Streamline] you won't need them for Gloom but Toxin Lash and [Venom Dose] still need shards to feel comfortable. Moreover, you don't need to use a Crimson Ability Strength shard to obtain max Gloom Slow if you choose to use [Power Drift] instead of [Cunning Drift].
Enjoy and have fun Tenno :)