Sneak attack now specifies that you can not use spells, and must use a finesse or ranged weapon attack.
Sneak Attack ... The attack must use a finesse or a ranged weapon. (PHB5e p.96)
However, spell attacks are subject to critical hits and count as an attack just like a non-spell attack does. The main difference is that you don't add your spellcasting ability modifier to the damage unless you have some abililty or the spell specifies that it does.
You're right. It's only base dice, no modifiers.
This probably sounds terrible, however, there is a good reason:
It's more balanced than it seems.
It's hard to tell sometimes; believe me I know. But, until you get more hands-on experience with the game, you have to give the game you're playing a "grace period" where you trust that the designers made the correct decisions, even if it seems like they didn't at first. Every campaign you play or DM is going to be different. If you change players or DM, then the campaign will be completely different. Some of those games, you will have players that built weak casters, and experienced grognards that built martial types. Although it is a roleplaying game, the "player skill" of the game comes in with decision-making during game time, and character building during creation.
That said, if you GM a game and you consistently encounter the same balance issues across encounters, then yeah, you should feel confident that houseruling whatever the issue is will improve your experience.
Your Wizard's spell slots are pretty big.
Bear with me a moment and imagine that the Wizard class is actually Batman. The biggest strength of the Wizard is that they have a tool ready for every situation -- at least, if they're a good Wizard. That's the player-skill-decision-making part. The spells your Wizard picked aren't bad, they just make him very good at things that aren't combat. Your Wizard has a ton of tools for dealing with non-combat situations. Imagine if Batman didn't bring his Batarangs when he went out on a mission. He's still the best detective on the planet, but now he isn't as effective as he would be in a fight. Spells are really, really good at solving specific problems.
That said, given the nature of the encounters you described, you might try throwing your Wizard a bone if you haven't already. D&D is more than just combat, and if you aren't giving your non-combat specialist Wizard any non-combat to specialize in, he'll feel like a useless player. If your player is concerned about his build, then you can allow him to switch out a cantrip, or give him a couple of good AOE scrolls in the next loot pile to scribe.
In addition, cantrips scale at certain levels. It's not immediately, but you can be sure that the cantrip will be a good standby at later levels.
Best Answer
Typeless Damage
Nothing in Pathfinder 2e puts the damage dealt by disintegrate as force damage, or as any other type of damage. This lack of damage type is repeated across a few spells, including implosion and power word kill, so it doesn't seem to be in error despite not being explicitly described in the list of damage types.
Damage without type would bypass most resistances, as they typically specify a subset of damage types like 'fire' or 'piercing' that typeless damage lacks. As you've mentioned however, an incorporeal creature's resistance to all damage would still apply normally, as typeless damage is still damage.