I have not found any Shield proficiency in the game. Can Wizards carry around a Steel Shield, and get +2 AC out of it?
What is the point of the Shield cantrip then? Twice the AC, same action. You can cast with a shield in your hand.
Sure, you cannot use Shield Block, but the cantrip allows it only once per combat anyway, after which you lose the AC too.
Shields (and their rules) are discussed on page 277 of the Core Rulebook. The Raise a Shield Action is on page 472.
You are correct to note that there is no shield proficiency and that anyone can both carry a shield and use the Raise a Shield Action. This requires buying a shield, occupying one hand with the shield, and spending an action to Raise it. You need to Raise a Shield every round if you want to keep the AC bonus.
The Shield cantrip similarly costs one action. So why bother? For one thing, Shield doesn't occupy your hands. That's worth paying attention to. Most mundane shields cost 1 bulk, which you save with the cantrip. And as you noted you gain Shield Block which you may not otherwise have.
If you do plan on Shield Blocking, remember you will need to budget for repairing or replacing your shield when it's broken. You will also need to continuously spend gold to upgrade your shield, keeping its hardness and HP adequate.
You already noted some of the limitations. Shield gives you a measly +1 AC, which is the same lousy bonus a buckler provides. On the other hand, the increasing hardness is quite nice.
So there is your trade off: a physical shield may offer better numerical benefits, but requires the bulk, gp, and hands to operate. If you are reliably taking attacks and the extra AC or shield blocking are important to you, you want a shield. If you only need that benefit in a pinch or if you need your hands/bulk/gp for something else, consider the cantrip.
tldr;Yes
The rules for wielded Shields don't list any restrictions or requirements for Raising a Shield, nor do the rules for the Raise a Shield Activity. However, not everyone has access to the Shield Block reaction - it is provided by a general feat, and some classes get free access to it, such as the Champion, the Fighter and the Warpriest doctrine for the Cleric.
The Shield cantrip uses only a verbal component to cast, meaning you don't need any hands free to use it. Unlike a physical shield, casting the spell grants the use of Shield Block, regardless of whether or not the caster has the Shield Block feat. Also unlike a physical shield, it can also block a Magic Missile. And although the cantrip can only block once per 10 minutes, the heightened version may well block more damage than a similarly-levelled physical shield.
The downsides to using a physical shield compared to the cantrip are the hand needed to wield it, as well as the encumbrance of the item, and having to repair or replace the shield. In contrast, the only real downsides to using the cantrip over the item are the inability to Shield Bash, half of the AC bonus, and only being able to block once per ten minutes. A physical shield is also still usable when the user is silenced, while the cantrip is still usable if the user can speak but not move.
Personally, I have a couple of PF(S)2 martial characters that use the Shield cantrip over a physical shield, due to being dual weapon wielders. I have also learned that the damage mitigation from a shield is usually more important than the AC bonus. Rarely does the AC bonus stop a hit, or lower a critical to a normal hit.
