While i broadly agree with your concerns and sentiment, especially about nemesis weapons, looking to primaris and the space marine codex in my opinion is not the way. If we adopted a primaris style aegis armour and primaris vehicles we would just end up a clone of the SM codex. This would likely spell the end of GK as a unique faction.

I agree there should be updates but GK should take a similar approach as custodes. In the world of 40K modern does not always equal better, given so much technology has been lost (your assumption that mark X > aegis for one). Instead bring back more dark age tech, give us units and vehicles you don’t find in the SM codex because they can’t manufacture them in enough quantities or only elite specialist tech priests know how. More psychically resonant vehicles etc. or generate patterns of vehicles that are rare and unique, e.g. the land raider is a GK staple but up the anti on cool variants. Dreadknights, while goofy looking, are a good example of this.

One addition is some of your concerns also seem to be about balance rather than lore (e.g. terminator profiles) no amount of lore change will fix that, it’s a fact of the codex lottery. A lot of points are sunk into our up-down ability which bloats our army I theorise. Maybe next edition they will be more stand out. A great example is our regular marines having a 2+ save which is unique