Some points
The artificer can turn any heavy armor into power armor. This means that they can turn that Plate Mail of Acid Resistance into their armor, and with the level 9 ability they can put infusions into it. I don't know what shenanigans this opens, but I think it is really cool to think about.
I'm a little shocked there isn't more discussion about the force field temp hp. It doesn't start as a lot, but with no limit and a set amount I see it as pretty powerful. I mean, I've been playing an Artillerist, and that 6-11 temp hp every turn has been incredible, and adding that to a build that can easily get 21 AC (plate mail, magic shield), I see that as really nice.
I definitely see Infiltrator lacking in comparison to the Guardian, it needs something a little better. Though, I will note, this is the only ability in the game that removes stealth disadvantage from heavy armor.
I do like the infiltrator level 15 ability though. Advantage with a 1d6 damage rider. Comes really late though, so maybe increase the damage a little? I'm not sure.
Spell Refueling Ring is really funny, because on a party member it can give you a 3rd level spell slot. On an Artificer it can give you a 6th level spell slot, which you can't even use. I'd almost be tempted to let them create a 6th level slot for giggles, but it is there.
Star druid is dripping flavor and I love it. I think the chalice ability is a little weak, but I like healing on my druids a lot.
Dragon though, holy crap. For 10 minutes you will not fail a Concentration check. If you have a +2 or +3 con, you end up being unable to get lower than a 12 or 13, meaning you need to take over 24 damage to even have a chance to fail. Building for Concentration checks and you are going to be able to guarantee concentration saves. That is huge
Cosmic omen is a little... odd. I'm not a fan but I don't know why.
Star Flare is just too weak. The teleport is great, drop it on your allies and you can move them up to 60 ft from where they were, redistributing the entire battlefield. But, it is a con save versus 4d10 with no damage on a success. This is a daily ability for level 14, at a minimum it should be save for half. And while blind is powerful, blind for 1 round is not that powerful. So, I'd really jump the damage up and make it save for half. The teleport is awesome, but this should hit hard and be awesome when the druid pulls it out.
I agree the ranger should have a scale to their psychic smites. I'm not against Dreadful strikes, but it does have problems.
Beguiling Twist is cool, I'm not sure how often it will happen though. Fear and Charm just don't seem to happen at my table, so I don't have a good judge for this. It is unlimited and it can work if you succeed the save, so that's cool. I think it might be fine, but I'm just not sure what to think.
Also, in terms of single target, Misty Prescence is incredible. They cannot see or hear you which opens some possibilities. And they only get to repeat the save if you hit them or cause them to save, so you can essentially designate a target who will ignore you in a fight. It could be better, but I think it is interesting.
