You want to avoid 1* star when you have access 5* stuffs because their health bonus is way lower.
To master autogun, you want to learn shield charge and learn enemy movements. It goes like monster attack -> avoid -> release charge. Or you could just shoot from faraway. Umbra driver is good. Alchemer charge deals heavy damage with a good bullet placement because the bullets can ricochet few times on same monster. You may want to look up how to switch shoot alchemer too. Magnus/Iron slug is fun weapon, the damage heavily based on charge. Since the knockback based on monster facing direction, you want a good aim.

Making Iron Slug? I think you can just buy recipes from hall of heroes and patiently heat up.

Your current set up is pretty much fine.. like, more than fine really- it's about the best you could realistically ask for offensively and pretty decent defensively as well, or at least without paying several million crowns for variants and whatnot. In fact, I used a very similar setup to yours earlier today.

You probably just need to heat your gear a bit more and get some good practice in more than anything else. After all, practice is basically free now that elevator costs are gone.

The iron slug is in no way hard to get, it starts at 3*, so you don't need the 2*crafting like most things, and from the hall of heroes, as already mentioned, so its easier to get than the chaingun you say you have

Also the gun boosts arn't so good because of no charge time reduction...

All of the 5-star versions of the new gunner armors have CTR+1 and ASI+1.

Some of them also have blanket handgun damage+1 against all enemies. Others have handgun damage+2 against just specific families. I always recommend blanket damage bonuses.

The only other choices to make are the damage protections (piercing, elemental, or shadow) and the status resistances (four options). Which one you pick depends on what your goals are. For example, if you want to use this set in Firestorm Citadel then Firefly (fire+, shock-) is the obvious status. You'll probably want either Sentinel (elemental and blanket+1), Shade (shadow and blanket+1), or Ghost (shadow and undead +2).

Well right now you already have the perfect mask of seerus, so you're basically set defensively for a lot of levels, and crafting more handguns first would benefit you more-- but that's just my opinion.

If I were you I'd roll unique variants onto your chaos cloak and mask of Seerus for the defensive benefits of that. Basically you'd be hoping to get either more fire/shock out of it, or a high/max damage resistance. If you must craft a new gunner armor, you would want to consider one for whatever enemy you have trouble with. If you have trouble with slags, fire+shadow. If you have trouble with poisonous slimes, poison+piercing. I would say to avoid the ice variants, and instead go for a 3*-4* black kat set when that event comes around, unless you just can't wait or really want a non-attack boosting gunner armor.