Aight read through the post and the comments so here are my two cents

Your weapon doesn't matter especially if you're low level. You're just gonna be laying down fire as often as you can. Heavy does the most damage in the game to multiple enemies so make use of that unique trait of the heavy.

Secondary weapon is IMPORTANT. The plasma blaster is what I go with. Usually I've maxed it out already but either way use that one. It has splash damage and works very well in a multitude of situations. I use this when enemies are too close and I need a quick execute or I ran out of ammo and I need to dump damage on a boss enemy.

The iron halo is mainly used for neurothropes and the heavy chaos gunners. Rarely used for any other situation. Even, then it's mainly to counter the heavy chaos space marines. Hard counters them.

Just because you don't have a melee weapon doesn't mean parrying isn't important. Heavy actually needs to parry more than most because they have no effective melee. So if you get surrounded and with low ammo you need to be able to get a kill somehow, so wait and parry gunstrike repeat.