Feedback is going to be piecemeal and somewhat all over the place... but hope to make a start. First class and first impressions...

Spellslinger

Between the first and third reading this has gone up a lot in my estimation and I like it a lot better than I did on the first read through.

So my reading of the class is that you are focussed on blasting - the class spell list seems mainly (with some exceptions) to be damaging spells or defensive spells.

I think you have possibly hit the same problem that I did with having a blasing option:

1) Blasting isn't always the best use of spell slots
2) Focussing on blasting means less versatility and the loss in power that entails
3) Blasting spells don't really scale well as an action usage

Fixing this then means you need AWESOME class features to compensate compared to other spellcasters.

I think that you have done this very smoothly. The 2/3rds casting progression means the gap between this and a full caster is actually pretty small. The gap opens up more at higher levels so you can dodge having to put too much in the lower levels and making it a dangerous dip for multiclassing.

Dex as the casting stat kind of makes sense for the class description but it doesn't really ring true for some spells. Throwing a firebolt makes sense with dex. Determining the con save for a moonbeam spell... less so. I wonder if you might keep Int as the casting stat but give an ability to allow dex to be used for any spell that uses an attack roll? That way you can still have a defensively strong caster if you want to go down that route (and all the advantage that can come from beeing good at hiding etc.).

Critical cast: Fine and it is not overpowered on average. My worry comes from a fun perspective; you have a non-trivial chance of just ending some fights in a way that critical hits on attacks don't as spells tend to pack more max damage per effect. Add to this that any magic resistance or legendary saves may feel doubly frustrating from this ability. I kind of wouldn't sweat it as a small ability, but just my thinking.

Aptitude - will look seperately

Spell sling - If I am reading this right, this is nuts. So you can cast Aganazaar's Scorcher (for example) from a second level spell slot. Then cast it again using the same slot and the same action... for 6d8 damage in an area of effect from a level 2 spell slot... Hit level 7 and you can fireball twice from a level 3 spell slot for 16d6 damage? Then maybe even add subclass bonusses on top. This seems so over the top and dominating in Tier 2 that I assume I am misreading it.

Fire at Will - Unlimited shield and absorb elements? I think this is a bit over the top in this regard. Maybe if it were limited to spells that need an action to cast it might be OK? Actually it is still probably a bit strong.

Limit breaker - only once per day, but it is still potentially pretty dangerous, not least because spell sling lets you cast two spells out of this slot. At the level you get it you can then be chromatic orbing or burning hands for 34d6 damage. Still, not as egregious as magic missile for obscene damage... and with the ability on classes like the hexblade/hexblade's curse to bust any instance of damage this could be exceptionally dangerous. Also, bear in mind that the Ravnica backgrounds and Eberron races can add unexpected spells to the class spell list so you might need to be somewhat prepared. Even something bland like Armour of Agathys from Mark of Warding dwarf becomes a real powerhouse. Or how about sleep for 25d8 worth of creatures falling asleep? Be careful of multiclassing or alternative spell lists is what I am saying.

Arcane wellspring - well everything is pretty broken at this level so there is a lot of lattitude. This is probably fine. Still, if you pick up spells like spiritual weapon from another class then you have a pretty fun massively powerful summoned weapon.

Limit smasher... hmm. Not much more powerful than 1st level spells on the spell list... but there are potentially some dangerous options available by multiclassing or from alternative spell list that could be good.

Limitless. Yeah, its powerful, but you need some spells that are crazy upcast.



Subclasses

Magneticist
A very cool subclass as a theme. I think there is a lot of fun stuff you could do here in play.

Spell list seems good but lightning arrow? I don't really get why this spell?

Magnetic propulstion: is this intended to let your spells of radius self be duplicated on allies? Or that you can pick an ally instead? If the latter then it is merely a very good ability. If the former then this is very much over the top.

Magnetic levitation: This looked pretty innocuous at first, but letting you levitate and drop people from great heights is potentially a bit unbalanced.

Magnetic Repulsion: When it gets used, this wil be a very powerful ability. It does rely on ranged weapon attacks being made.

Magnetic Impact: This is very open to interpretation. Can you push someone down stairs and they take Int damage every step of the way? It is like a free concentration free spike growth.


Thermalist
I worry about this as it focusses on two damage types that a whole class of fiends are resistant to. I mean, I guess its no worse that something like a fire focussed dragon sorcerer... except you get many fewer options for non damage spells to interact with the enemy.

The multi element spells on the spell list don't scream thermal to me either - a mix of hot and cold spells would seem a better fit?

Prolonged exposure - at higher levels the extra damage is less likely to be critical, at lower levels this will be very powerful. Given the scope to have spells hit multiple creatures, and given there is no concentration need, no activation cost, limit to uses per day etc. it seems very strong.

Antipodal Anihilation - This has me really worried about my interpretation of spell sling - hit with one spell then your next spell on the same turn deals double damage? Also, is there a time limit for this? I minute? A day? Till the next attack?

Thermal Balancing: How is this supposed to work with damage immunity? With saves for half damage? Temporary HP etc. Other damage reduction?

Temperature compensation: In reality this is probably just immunity to two elements. THe number of enemies that can do both damage type is pretty limited. Not that this is a problem; it is a nice thematic ability and immunity at this level is... a bit early but not that egregious.



Vulcanist

Another cool subclass and it does somewhat take the class in a different direction with some more controlish spells.

Lava bender: is "smashing damage" bludgeoning or force? Beware of the scribe wizard multiclass though for magic missile shenanigans. Also it makes this an attractive dip for something like a battlesmith (if bludgeoning damage from weapons gets added to).

Magmatic Emission: What defines an "area" a 5ft square? A contiguous effect from the same spell? If every 5ft square counts then spike growth is a really big deal. Like problematic... With a weaker interpretation this still gives a very nice bonus to some powerful effects.

Stoneform: fine.

Worldbreaker: Wow. This is crazy. Multiple free castings, especially with recovery of the spellbreaker ability AND extra damage AND you get it early AND you get a lot of bonus effects that apply to this?




All in all I love the theme of the class and I like a lot of the mechanics but I think it is pushing too many good abilities at too many levels. Not many dead levels. No ribon abilities... and some of the abilities are insanely powerful with so many interactions. Not being a full caster is a major cost - but a number of abilities just totally circumvent that (casting multiple spells from the same spell slot, casting spells without spell slots). Really in practice I think the only weakness is the lack of versatility in the spell list - and this is fixed too well by the character options that give you access to more abd by multiclassing.

Some of this may be my misinterpretation but I think that this needs a round of curation to clarify some wording and curtail its power - especially in tiers 2 to 3, but for a Beta version this looks pretty sweet.