General

Dual Blade- low dps unfunded but amazing dps funded

get a sw katara and perf that and the main weapon. It picks up speed once you perfect both weapons. Until then the damage is subpar but I highly reccommend playing one and is very rewarding if you fund it. They have very good burst damage with blade tempest and asura and on top of that they have great mobility, amazing burst damage, and plenty of iframes with coh and fc (coh is broken lmao). You also have innate lifesteal, 100% crit, and mirror dummy gives aggro control and procs shadow meld (which gives 100% crit).

I'm not gonna lie to you like everyone else, unless you commit to it heavy dual blade is going to be weak af.

You gotta go hard with the luk% and att%, no ifs and buts about it.

If you not planning to give it your all and get only minimal boosts in range until after a certain point, quit now. You won't feel any change as a dualblade till you've hit 2m.

"I only got my class to level 171 and it isn't good! whaaa whaaaaa~~" Your giving up way too early man.
Have you tried scrolling your items?
Have you tried Star Forcing your gear?
Have you applied potentials and bonus potentials? (probably the most important factor in range nowadays)
Have you tried getting the boss set? (zakum, HT, magnus, PB etc. accesories. They're good for early range)
Have you started working on gollux sets?
Do you have link skills and proper character card decks?
RA Katara is kinda useless. You also mention only having 3 set bonus, you need that 4 set. Hat/Top/Bottom/Weapon.
Replace RA Katara with Tsukoyomi (japanese Katara) until you can perfect that, then transpose to SW Katara.
Again. Your level 171. Don't expect 10m range handed on a platter lmao. 100k isn't awful for your level.
1b is enough to either start investing in merching or buy SW face+eye acc.
DBs are very enjoyable. They have several valuable skills to add to their mobbing and bossing experience, with more coming in Beyond. Their new 5th job skill is arguable one of the best if you can land the entirety of it for mass burst damage.

@corvae: I would pick Aran. Arans got a decent revamp (not sure if anything will happen in later patches), but I like the way Aran is right now. You can use combination keys to use skills or assign the skills to your keyboard. So Aran isn't that hard to play. They are pretty strong with low range, very mobile for a warrior.

Evan is strong too but Evan depends a lot on Mir's fusion skills. I find Evan plays a bit too stiff. So I don't enjoy playing it. I don't mind leveling Evan to Lv200 but I will never main it.

People said Hayato is weak, I don't know but Hayato is great in mobbing, and is also very mobile. They should be good for training.

I can't give you a lot of details as in damage, fundings. I didn't main any of them.

So what is your main goal when playing this game? Sure 80k-90k range on your DB is low, you can beat normal Zakum, Hilla, Horntail, Root Abyss, Krexel, Easy Magnus. Do you want to beat the more difficult bosses in the game? For any job, you will need a lot more range, even 300k range won't do it, sure you will be able to add some bosses to the list that you can defeat but still restricted to the same bosses.

@corvae: You might wanna go with a number. Medium finding could be 5bil, depending on who you ask

@hualicious: Hmm, out of evan, aran and hayato which would you say is the strongest or most fun? (Medium amounts of funding)

@corvae: awh if you don't find DB fun anymore, then there is no point in spending more time on it. Personally I love DB. Archers and Pirates are among the cheapest class in MS. Try to see if you find any of these jobs fun. But still, they also require some fundings to enjoy more of the end game content.

@hualicious: honestly, i've been sceptical about still playing dual blade since I just stopped finding it fun and only play it purely due to how much effort I put into it. What are some fun / cheap classes to play in 2017? Pains me to see that im going no where with my db

With that range you can get to Lv200 without any problems, however you do need more than 1bil mesos to raise your range by a lot. At least you got something to start with. Try to earn more mesos, there are guides for that and see what fits you the best.