So, this is just a few ways to minimize the amount of money you spend, and maximize the value of what you do decide to buy. This is based primarily on my experience playing on the JP servers for the past few years, so features that are currently NA exclusive, such as the Mission Pass, are not accounted for, and some monetization schemes may have been altered between versions and I haven't noticed yet.
And as a side note, if you like the game and want to support its development, absolutely spend a responsible amount of money to do so. This is only meant to help those who either can't or don't want to spend money get the most out of the game still.
And with that out of the way, let's start freeloading!
The first thing you should do is make multiple characters. You don't need to actually PLAY all of these characters right away, but you should MAKE them. The first reason for this is to triple the amount of items you get from character specific campaigns/rewards. One of the biggest things you'll be looking to get out of this is the All Skill Tree Reset Pass, traditionally released on JP alongside balance adjustments, allowing you to respec without spending AC. Another major benefit of having multiple characters is taking advantage of the additional character storage space. Default storage is shared between characters, while character storage is not. With three characters you have significantly more storage capacity, and you can shuffle items around as needed to get them to whoever needs them.
After you get used to the flow of everything and feel comfortable with your main character, it's time to start putting your extra characters to work too. Make sure each one is in a different category of class (One melee, one ranged, one tech, or sub any one of those out for a hybrid class (Braver or Bouncer)). Get each one to level 50 as soon as possible. This unlocks SH difficulty and the best ARKS Missions. Do your dailies and weeklies on each, and you will be able to get over 6,000,000 Meseta per week, which is what will be funding your fashion without access to the AC Scratches. You should build your Mag in accordance with each character's main class. Unfortunately, three characters isn't enough to cover all four Mag types you might want, but each should be kept pure. Ideally you want one for each attack type, then one for Dex. Up to you which one you decide to sacrifice, though I recommend either Ranged or Tech to be the one you leave out. One last note on Mags is that if you find you really need the one you gave up earlier, buy another Mag for 300AC. This is cheaper than buying an extra character, or resetting an existing Mag, and will be the only AC purchase in this guide.
Next up is efficient use of your secondary currencies, SG and FUN. FUN is pretty easy to come by, you can get it from logging in, reviving other players, using the lookbook, sending Good Jobs, etc. Basically anything involving interacting with and helping other players will grant FUN. SG is a bit more work. Unfortunately, most of the renewable sources of SG on JP seem to not exist on NA yet, but you can still get a solid income from your Main ARKS Missions and Title rewards, as well as from playing through the Story. So make sure you're doing those!
FUN should be mostly spent on FUN scratches. You'll get a few generic fashion items, but the main thing we're after here is the My Shop Pass. This only lasts for three days each, BUT anything you have already listed on the player shop will stay listed and you can continue to receive Meseta from sales. So before using each pass, make sure you have a lot of junk to sell, and list them all at once to get the most out of your "three days." I'm afraid I'm not the one to ask for the intricacies of profiting off the player markets, but the extra income will be appreciated even with just tossing a bunch of junk on it at the recommended prices. Your first priority with SG is Material Storage. I cannot overstate how useful this is. You have functionally unlimited space for all order items and materials, which will free up a TON of inventory and storage space. Also you can spend these directly from Material Storage, without the need to move them into your inventory first. Get Material Storage before anything else with SG. After that, check the Fresh Finds Shop. There will frequently be unique fashion items, and more powerful Triboosts/EX Boosters, which will ease the burden of leveling quite a bit.
And that should be all you need to keep freeloading for a very long time! If I've made any mistakes here, or you have any other suggestions, feel free to add to this guide! I'll also check in and answer any questions anyone may have, so post those too! Thanks for reading!