Greetings, stranger! 
Here I thought I was the only one doing the Stardew Valley/Grim Dawn combo… (Apologies if somehow there are two Giant Octopodes on the internet).
Many points:
- Pyromancer is Demolitionist+Occultist, this is a Sorcerer.
- The concept of “build” in this game very much does include your exact gear setup. Particularly once you have AoM (and you pretty clearly do since you picked an FG devotion in your setup) items can make major changes to skills. If you’re going for fire AAR, you’ll be wanting something to convert the aether to fire, for example. The other reason “build” doesn’t really mean anything without the gear is that there are a lot of sources of +skills, so in practice you’ll have a lot more skill points to play with than you think.
- If you haven’t already, a thorough reading of the game guide would help. For example, Ghoul doesn’t help AAR builds, since AAR has no weapon damage and thus doesn’t get attack damage converted to health (ADCtH) from Ghoul. It can get ADCtH from a certain off-hand. And there’s an item that can add weapon damage to AAR, in which case Ghoul can be viable with AAR (just to be clear, Ghoul is arguably the standard survival devotion for most builds in this game - just not the pure caster builds with no weapon damage to their main skill(s)).
- Likewise, flat damage from buff skills like Reckless Power can be very strong, but doesn’t help you at all with skills with no weapon damage like AAR. Reckless Power still does boost you a lot and is well worth the points, but just be aware that what you’re getting is cast speed and the +% fire and aether damage, not the big chunk of flat aether damage on top.
- Regarding AAR, it is now a great skill (I remember back in vanilla when it was what the kids these days call a “meme” skill). You’d definitely want Mental Alacrity, but people typically don’t invest much in IEE. AAR builds do emphasize energy regain in their devotion setups. Arcanist skills alone aren’t nearly enough to keep you going, and you’ll never be able to ignore energy management with an AAR character.
- I don’t think you yet understand quite how important resistance reduction (RR) is in this game. You have some (thermite mines), which is already better than a lot of new players do.
But the correct amount of RR is “all of it that you can possibly get for your damage type”. Particularly the kind phrased as “-X% Fire Resistance” since those stack and the other kinds do not. Solael’s Witchblade is actually the most important devotion for fire builds, and one will usually want to get Rhowan’s Crown and Viper as well. - Conversely, your own resistances are a hugely important part of your defenses. Keep them as high as possible (capped if at all possible, which it should be from ~level 30ish or so). That was my first and most memorable learning experience back when I started playing - character deaths dropped precipitously as soon as I made resistances my absolute top priority.
- OFF is a newb trap skill. It can be great for clearing trash, but in this game trash is meaningless, no amount of trash mobs can ever threaten your character. Bosses (and nemeses) can, however, and guess who is mostly or entirely immune to CC effects like being frozen? And just to be clear, if an enemy isn’t frozen, it doesn’t get that -100% Fire Resistance either. Apparently there’s a specific level 94 set that can make OFF work against bosses/nemeses, but I don’t think that’s what you’re aiming for. If you like, OFF plus the fire blast skill from a component is a common suggestion for early leveling with an arcanist. I don’t like it, but your tastes may vary.
- Survival skills - Blast Shield is nice, but won’t keep you alive by itself. Maiven’s Sphere of Protection is an excellent defensive layer, and Mirror of Ereoctes is the best active defense in the game. Don’t expect to be able to facetank dangerous opponents, though.
- Worth mentioning that Nullification is a great quality of life skill. Being able to clear an enemy’s buffs, and clear your own debuffs, is very handy. As you’ll learn from a few enemies who can do it to you (try not to let them). Not necessarily worth maxing out, but throw a point in there just to have it as an option.
- Arcanists pretty much always want Inner Focus for the spirit and OA. Also note the OA and aether resist on Overload now. And Demolitionist has OA on Flame Touched and DA on Temper (a fire build would probably skimp on the latter since you don’t get anything useful other than the DA).
- Looking at the Build Compendium is a good way to get a sense of what good builds look like. There is one doing the fire AAR plus OFF combo you’re looking at. Good luck finding the four pieces of the Mageslayer set, though. The weapon converting aether to fire on AAR is crafted, so you can get that the moment you hit 94. What you do to get to 94…
People on this forum love to suggest leveling one way, then doing a complete respec at the end to the actual build they want to play. I find that utterly bonkers, but that also means I’m not willing to play (or even consider) builds that are heavily dependent on very specific gear effects like this one. - Attributes - the rule of thumb is still all physique, maybe a few points in cunning or spirit if needed to equip pieces of gear, though I don’t think you’d need any with a sorcerer. After the changes a couple(?) years ago it is viable to do spirit-dumps to some extent, but that’s for people who know what they’re doing and want to make their builds glassier for more dps. Builds that do pierce, physical, or bleed damage can put a lot of points in cunning if they want, but that’s not relevant to a fire sorcerer.
Anyway, I hope this helps. Fire AAR sorcerer with OFF can be a thing, but I would not recommend it for a first character. No harm in playing with it, level for a while, see what skills you do and don’t like. The real strength of Grim Dawn is the huge build diversity, so create as many alts as you like (says the confirmed altoholic… ). Sometimes a nice gear drop will result in creating a new character just to use it.
For what it’s worth, speaking for myself, if I wanted to do a fire channeling skill, I’d do Flames of Ignaffar rather than fire AAR. I’ve never actually done that (my aether AAR spellbinder is enough of that playstyle for now), but as far as I know it’s perfectly viable.
Like I said, some time spent browsing the build compendium can help a lot with ideas, as well as learning what to do and what not to do. There are a lot of beginner guides these days that are particularly helpful. They produce characters that are easy to gear effectively, and have more explanation of why certain choices were or were not taken.