I've used both my umbra forma on him and have been having a lot of fun. I have 210% power strength and lose some duration for efficiency with prime continuity, transient fortitude and fleeting expertise (last 2 not maxed). I'm thinking of getting a second over extend to to get enough power strength to keep 40% damage reduction on serene storm but put some range on elude and lull. Since I have 90% duration lull finish a tiny bit faster, which let me spam it bit more. I'm also pretty much immortal, desolate hand 90%, adaptation, restraint 0-50%, and serene storm 40% damage reduction with umbra mods make for an insane level of survival. Add in elude and profit.
His only real downside is not so much removing(? going from full to 0 in order to be able to use serene storm) but losing it too quickly. Something that might work is to decrease serene storm passive restraint gain and put it on certain part of combos based on range and power. So you if you spam into the void, you're wasting restraint but if you only use it when there's a good group of enemies for all the aoe and use primary/secondary for solo target you would be able to manage restraint much better.
Another option is to have restraint gain/loss affected by efficiency. I'm also all for an increase in restraint loss from lull, possibly make it have multiple tick per enemies instead of all at the end when it fully sleep the enemy. Which is what it does in my understanding since until I read it did remove it on the wiki I had never noticed it once. Enemies tend to die before the sleep hit, especially in a squad. Something else that could work with the calm buddha/angry buddha is to add a passive loss of restraint whenever an ally (pet, other tenno, defense objective?) get hit. Something very small obviously and should have the affinity range.
As for direct ability buff, I would probably touch a bit on the range and rythm of throwing desolate hand daggers and maybe tweak the base number/damage reduction. I'd like it to have a clear boundary of "if an enemy get in this zone, I will throw a dagger". All in order to help a gameplay where you jump in the enemy throw tons of daggers and lose restraint, back up get some daggers up and stay at medium range to keep the damage reduction. Also, giving daggers to ally should duplicate them and not remove them. Some more interaction between abilities could be really nice too, recover daggers when close to lulled enemies?
TL;DR: Baruuk is cool and so close to being there. He doesn't really lack any damage as many people point out, but a little bit of fluidity.