My first build was pure PSI, and it was a lot of fun. I can't speak for tranquility users, but psychosis made everything interesting. There was no lack of engagement there. You went into every scenario thinking "How am I going to survive with so little hp?", then worked out a plan and just wrecked everyone. I'm sure some people went into a lot of fights just using a few OP abilities over and over, but you had so many options with that build I can't imagine many people who ran it never experimented. Occasionally you'd have to do a bunch of savescumming, but that's what you got for going with a glass cannon.
It will be a bit harder with the new PSI, but I don't think the changes are too much. I've reworked my old build and the biggest issue I can see so far is the psi cost increase from multiple schools. Despite the stated intention, it does seem as if it were put in there just to punish you for going with multiple schools. The amount of slots are alright when you're high INT, but limiting them this way will definitely push people into sticking with OP abilities more often and result in less experimenting generally. This was one of the best parts of being pure PSI in my opinion and it's a shame it's been so neutered. The topic "An Average Explanation of Outstanding Psi Builds" reflects this, boiling down combat to "Use Metathermics, you only need 3 or 4 abilities."
Skill points are made much more abundant for pure PSI builds by the high INT requirement, so adding PSI slots to INT as well seems a little counterproductive to me, though logically it makes sense. To make it harder to spec into PSI for the advantages it gives, wouldn't it be better to either lessen the negative effects of overloading their mind through either some restrictive gear (for example a belt, removing the ability to also use a doctor's pouch, or even just another headband component) or maybe simply through being able to specialize psycho-neural flexibility? Removing the limitations outright would defeat the purpose of the update, it would seem, but as some feats reduce them already, it seems logical to me to suggest lessening the restrictions more though either Advanced PSI Empathy or just an entirely new feat since a lot of pure PSI would likely take that anyways. Currently the feats seem designed to gently prod people into innervating one or at most two schools. Since these builds have no lack of feats to choose from already, making that choice more difficult would be a good way to restrict them. As for making it harder to allocate skill points instead, PSI Empathy could always just have some line such as: "Focusing the mind on the development of its psionic potential has its downsides, namely the inability to simultaneously focus on other areas of study." making INT no longer give bonuses to other skills. That would cut out quite a few of the extra skill points.
PSI reserves are a nice addition. The only time it gets annoying is when you do want to experiment, since innervating a new set of abilities so frequently leaves you consuming inhalants left and right. I see the reasoning behind it, but it also seems odd to me that this mental energy is incapable of replenishing without a man-made medicine. Maybe you know what I'm going to say... a feat could solve that. It could possibly be added into something like Forceful Innervation, and even if that never happens, it would still be nice to be able to unlock psionics that way instead of the pill Pasquale gives you. I still remember doing the first few quests without psionics before looking up a guide and finding out that "natural way" didn't actually exist. Forceful Innervation vs Medically induced would be an interesting fork in the PSI path similar to Tranquility vs Psychosis.
More than anything I miss the flexibility of a build chosen specifically for it, and I hope in the future it will be restored to a greater degree, even if that comes at a great cost to other convenient parts of the build. It was nice knowing you could stumble into situations like the one pictured and still be able to navigate them without too much frustration. All in all I think the changes are interesting if a bit convoluted, but I'm glad to have a game where you can come back after a few years to find such complex updates as these, even if they sometimes derail you. I hope the release of the legacy branch doesn't mark the end of PSI's fine tuning.