False. All FW
Knight will still sports a 3++ (and they're not game breaking), whilst Castellan will probably be over-nerfed defensively since it will have a 5++ upgradable to 4++ by 3CP. And for 1-2 CP
you have a 2+ relic... But I think this was a sore-thumb post more than a productive one.
I honestly like the idea, but following the current rules for Void Shield. A degrading invulnerable save from Void Shield will probably feel better on the Knight, as long as the Mechanicus stratagems for full efficiency specify that doesn't work on the saves, and you remove one of the stat that goes down (Movement, probably, even if in the case of the Gallant the BS
may be removed from the does degrading profile since it does not use it). Correct me if I'm wrong, but not model under 8th rules have more or less that 3 reducing characteristics (and 3 degrading profile, for what's worth). It will be bad to house-rules one of the few game elements un which they have been consistent.
Another interaction is related to Mortal Wound. Void Shield blocks them, so the Knight generally speaking lose as much as they gain because you remove one their liability. Currently in the entire Knight Codex there is a Mechanicus Knight stratagem for a 5+++ during a phase, while Hawkshroud (Imperial) has a relic for a 4+++ but only during psychic phase. Both can still work.
Gallant will result probably overpowered with a "real" Void Shield mechanics
because you have almost no chance of degrading me in the only turn you have (if you're lucky) before I impact your front line.
So, it may works almost as-it-is under the current rules, but changes very very little. Your beef is not with the 3++, it's with a specific unit (the Castellan) with a specific setup. In my Knights list I don't ever used the Ion Bulwark, it's overkill almost all the time.
That said, a 3++ vs a 4++ means, in reality, an adjusted wound count of 47 / 42 (on a Castellan) or a 40 / 36 on another Knight.
Your problem cannot be 5/4 extra wound when you have to already inflict north of 40 damage. A difference around 10% is weep within the standard distribution of rolls.
The 6+++ alone impact almost the same as total wound, but is even better because works against Mortal Wound and all small fire (while Invulnerable is redundant with armour). And the Household wiith 6+++ is Mechanicus and have a respawn stratagem... plus the stratagem to fight at full efficiency. Yet none seems to play it, it's Raven all the time.
The really problematic stratagem is Order of Companion applied on a relic. It's ok on a battle cannon (makes it at least usable),it's overkill on Cawl Wrath. That' it. If I may apply the Order of Companion to the Thunder of Voltoris (house Terryn relic battle cannon) it will be also broken as hell.
I have even wondered if the interaction between Order of Companion and the Blessed by the Sacristan trait (a not-relic weapon inflict an adjusted extra Mortal Wound on a Wound roll of 6) has been considered.
But, really... it's 1 household-specific stratagem. No need to tinker with anything else.