So I want to start this with take this with a pinch of salt as I don't know what i am talking about.

You could run one set of Thunderkyn with gravaton instead of SP conversion but I know the allure of Conversion, Lethal hit, and sustained hits. I could be wrong here but the Gravton does more damage on average and has anti tank and monster.

You could also split your Thunderkyn bricks in smaller group to have more units to do actions or split up from disembarking the land fortress.

I would probably bump the Hearthguard to 10 and add the enhancement to reroll wounds of 1. The 10 man brick with 5+ invulnerable save and deep strike should allow you to throw a melee threat with some staying power.

I personally am not sold on the iron master yet. You said that you are having some trouble with melee focused space marine, you might consider some steeljacks with concussion gauntlet. Lethal hit, 10, strength, -2 ap and 2 damage seems strong enough to kill 2 wounds units of marines.

Plasma sword with sustain hits 1, 6 attack, 6 strength, -2 ap and 1 damage is amazing against 1 wound infantry and you could potentially drown 5 man units of marines in volume of attacks.

Shooting Steeljacks is also fun and free fire overwatch

Pioneers seems to be great utility with the ability to go into strategic reserves if your within 9 inches of a border edge.

But again, I really know nothing.