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.