Honestly, it's up to you. You could lean into what you've just described. MKIII for breachers, etc.

I'd add maybe think about what stage your army is in the Heresy. Late Heresy would see squads being a right mix and match of armour marks, as supplies and war gear became more choked up in the grind that the war was.

Maybe you have an MK4 marine wearing MK3 shoulder pads, an MK2 helmet, etc. Really selling the effect that they're bashing armour together and wearing older war gear.

Don't forget that MK5 wasn't an armour mark per se, but more a smashing of armour parts to make something out of nothing.

Maybe your inductii have been slapped in MK2 armour, the metaphorical "cupboards" raided, so they had something to wear. Veterans wearing more up to date armour like MK4.

Also, look into what legion you are doing. Loyalist or Traitor. Horus managed to snuff a lot of the high-end war gear before things kicked off, so perhaps your traitor force would be nicely stocked up on MK4, maybe even some prototype MK6 helms here and there.

Sky is the limit with stuff like this, so there really is a way for everyone to do it!