I feel like Mightnight-Dj gave you a really good answer, but imho he gave you much more information that you actually need. Let me try do it real nice and fast. Increasing survivability - you should just try to dodge everything/stay outside of the range, use charge attacks for easy hit-and-run approach. Have trouble soloing a mission? Try lowering the difficulty or playing in a party, even tho monsters will have more health, their numbers will stay the same, so you'd not be overwhelmed so easily If there would be more players to spread the love. Getting better gear is always a good idea. The cobalt set you're using, it's... how do i say it. In every mmorpg you get that first starter set or at least few items to start with. Treat it like that. There are definetly better options, but this game is more about gearing up to counter specific monster type/to have an easier time in that specific scenario, you will most likely not use one set or one weapon for everything. Nonetheless you can heat it up, it's not only required for upgrade to 4 stars, but it also increases it's defense/attack. You also gain bonus charge time reduction for lvling weapons. Spending $$$ only speeds up your progress, there's currently nothing you can get for $ (outside of cosmetic items) that you cant buy from in-game currency farmed. You can freely exchange crowns into energy and vice versa, buying for example alchemy orbs for energy is nice and fast, but these also drop in-game so can be obtained without a use of credit card. You will gain crowns just from playing the game. Whenever you go, there are crowns to be obtained.

And also, getting into a guild would be a good idea. You could have someone to ask for help at all times :) Community in this game is definetly one of the best out there.

Here's the short answer:

Cobalt/azure guardian is inferior to basically any weapon-bonus armor: directly so to those which also sport piercing defense. All Azure has is a low normal defense bonus, at the cost of usually a medium weapon bonus and a full bar of status resist (see wolver/gunslinger/demo armors).

Normal damage weapons are bad. Unless your weapon's base damage is so high that you kill everything in a single strike anyways, you want to use an efficient damage type. Use two different types and, in the worst case scenario, you'll still have a weapon that's neutral against whatever you're facing.

Here's everything you need to know about what items are worthwhile:

• Mercurial demo has a movement speed bonus and medium bomb damage. With the right UVs you can get max bomb damage against two monster types per weapon and max charge time reduction with it. Bombs don't have/can't get attack speed UVs anyways. It's a shadow lair item though so you won't get it anytime soon.
• Black Kat has a movement speed bonus and max damage for everything, against everything. It's generally considered the best armor in the game, except maybe for bombs. It's an event item so you won't get it anytime soon.
• Chaos set is discount black kat. Instead of movement speed and a max (+6) damage bonus, it has very high (+4) bonuses to damage and charge time reduction. If you have a very high charge time variant, the CTR is pointless (unless you're using bombs with the striker shield in lockdown), but you probably won't get one of those anytime soon. This is much more obtainable than the previous two sets, but it only has a 5* version.
• Almost anything with weapon bonuses is good. Except Mad Bomber. It's the same as chaos except it only works for bombs. The only advantage is that it lacks a curse weakness.
• Almost anything with monster bonuses is bad. Except, a medium fiend bonus will let you kill Trojans in a single blitz needle charge in tier 3. The helmet for the divine set has this, even though the armor lacks it for some reason. This is unnecessary if you already have a medium gun bonus or medium damage bonus from chaos, but it's kinda common to see it paired with the Sklover coat for well-roundedness. Much like chaos, it only exists as a 5* item; they both come from the magic hood.
• Defensive sets are kinda bad. For example, in PvP, with a Final Flourish (pure piercing weapon), a player with 1 Sklover and 1 Justifier piece will do almost the same damage to a Royal Jelly player as the Royal Jelly player does back to them with the weapon. That's just one medium sword bonus, and they're tied. If you add max pierce UVs to each player, or just use full Sklover for more bonus, Jelly starts to lose. Plus, defense won't make you money any faster.

Also, don't mix and match defense types for lockdown. Any defense you have below 100, just gets brought up to 100. Even a Royal Jelly piece with a max pierce UV is only 101, so if you're mixing four different defense types together, you'll just end up with less defense than someone who stuck with two.

For weapons, Blitz needle is great for Trojans and firestorm citadel, and in-general. Grim Repeater is the same thing. but for dark damage, so it's great for the tier 2 boss dungeons instead. Electron Charge or any other grav bomb, when combine with any weapon that has a crowd-penetrating attack (like magnus guns, brandish swords) is good. Shivermist will make the game easy, but stops grav bombs from working. Alchemers are great guns if you can use them. Mixmaster and orbitgun are probably the best weapons in the game, but cost millions of crowns to obtain. Final Flourish or the Barborous Thorn Blade are great, especially for lockdown. Acheron (a brandish type sword) is also good there, but you should also have a gun.

For shields, if you end up using the swiftstrike buckler shield, you'll need to grab a 5* ancient plate shield from Brinks to pass the 4 and 5* verification quests. It's a decent shield and also the most efficient item from Brinks that you can sell back to other NPCs for cash (tied with the rest of the ancient plate set). Otherwise don't sweat the details.

I was also contemplating throwing in a piercing weapon but I'm not sure if a third weapon is worth the money.

Since you are in 6-1, your next boss is going to be Roamulus twins, a construct boss within a 3 level dungeon similar to RJP. Since all enemies inside are either resistant to piercing (construct and slimes) or neutral (Grimlins). Piercing guns won't do you much good there. That been said, you should start looking into Blitz needle line, you will need it against beasts and fiends, and for the final boss of the game...

Exactly how far in the story do you guys recommend going?

Depends on how long you want to stick to the game. If you just want to unlock the place that you, and most players will be grinding the most, then 9-4 will suffice. The maximum rank currently is 10-3, but most of it is just tying up the loose ends in the story with the exception of one mission. Oh as a spoiler, there won't be any satisfying conclusion to the story and a lot of questions will go unanswered. GH seems to have given up on developing the lore.

I'm half tempted to suggest you go for the cryotech rather than the voltech, just because cryo is the only elemental-freeze gun while for shock there are several options. Both are fine though, I just use nova. I would really, reeeally recommend getting needles though.

Jelly palace/roarm twins are your best bet for money now. Later on you'll want to play firestorm citadel, which gets you around 8.5k per run plus 3 boss tokens. They're worth about 1k crowns each since you can buy the ancient plate shield for 30 tokens and sell it back for 30000.

Samaliel, since you're now asking about earning money, you might want to check out the Lancer Knightz mission payout data sets and Skepticraven's Arcade payout data set. They give you lots of insight.