...Why not drop the power armor idea(at least for ourselves - our most experienced subordinates could use it) and make a normal, light, non-restrictive armor, and then cover it in runes and crystals that embody the ideas of deflection/blocking/damage resistance, while also using sorceries, seer knowledge and psychic abilities to boost the effects until we have a constant LOLnope field around us, that only the most powerful of opponents have any hope of getting past? Or an armor covered in Psyker-amplifying crystal that turns a candle-igniting trick into a continent-blasting fire nuke, and a continent-blasting fire nuke into a portable Sector-range Exterminatus.
It'd be FAR too expensive and take WAY too much effort for it for it to ever be mass-producible, or available to anyone who isn't us(or the Emperor later on, maybe), but that's the exact kind of Primarch-made, unique, personalized bullshit gear that legends are written about, and last for 10k years.
Of course, we don't have the skills for that yet - we barely just started researching Runes and Crystals, and we have little to no Sorcerous knowledge at this time, but that will change, and the personalized equipment of a freaking PRIMARCH deserves to be absolutely TOP-GRADE. No mere power armor will ever suffice.