First suggestion I'd offer is to dig through these forums for existing posts to gather info and insight, particularly the "Ask a Veteran AKRS" thread. Lot of great Q&A there to start with. But, a quick rundown of what I've learned:
I've got three characters going right now, a Ranger, a Braver, and a Force, and I've focused mainly on distance fighting (being a solo player as well I figure the farther away I am form the enemy the less likely I am to get taken down). Ranger definitely offers the greater distance as well as, of the three, the better "strafing fire" (ability to attack while still moving). I'd categorize it as a "slow jog." Braver comes in second in that category, a "walking" strafe while holding the bow drawn. And force is a bit of a slower "walk" while charging a spell and has to stop entirely when casting.
For damage output I'd say Braver delivers the most per hit but I've struggled with accuracy due to the arrow needing to travel, followed by Ranger with their 3-round burst (near instant hit registry, no "travel" for normal shots). Honestly not sure exactly where Force would fit in as their offensive power is dependent entirely on their Techs, "normal" attack power is fairly negligible. Still, with the right tech combos they're pretty heavy hitters (combos like Shifta to boost attack, Negiverse to add a healing effect when you deal damage, followed up with Ilgrants for a decent damage dealer with homing capabilities and multiple hits to draw multiple heals.
For most classes the go-to subclass is Hunter, with many focusing their attention on "Fury Stance." It gives a boost to range and melee attack at the cost of being more susceptible to melee damage. In essence, the "glass cannon" approach. Does a lot of damage, just don't get hit. For something like the Force you'd pair it with the Techer/Techter. Basically you want to find a subclass that meshes well with the main class. "All in" rather than "jack of all trades."
On that note: Mags. It's all in. If you're doing a Ranger, put everything into RNG ATK. If you're a Fighter, all on MEL ATK, Force or Techer, all on TEC ATK. Don't put anything into any other stats. Braver and Bouncer, however, are slightly less obvious. You want to place all bets on DEX as these two classes apply DEX universally to their respective attacks (Braver: MEL and RNG, Bouncer: MEL and TEC). This will maximize your DPS. But, again, "just don't get hit." Focus on bolstering defense via Units.
Auxiliaries can do "collect/procure" type Client Orders for you. In fact, sending them off on searches is the only way to level them up, so start them off on some low-level stuff and work them up. You can bring them along as extra help on missions but you're better off going to the right-most tab and picking up some "Player AI" companions. You'll get 5 "FUN" points for each one you have with you and for each mission you complete, and FUN scratch tickets have a chance of giving you 3-day access to your Personal Shop. You can add items to your shop during that time, change prices, etc. Once the pass expires your shop is still open and you can still get Meseta from sales, you just can't tweak the inventory.