I've been thinking about something like this myself except the 3 action variant was a 5ft bubble of shield around you giving others inside it the bonus as well.
That's actually pretty cool! Haven't thought about that one but I love the thematic around it, and it improves team work which is always fun.
Way too strong imo ( especially while quickened ).(...)
Hmmm, yes, possibly.
Yeah, the 1 action is finely balanced with regular shield raise shield. But the 2-action VS for +2 is straight up mechanically worse, it's there for its magical flexibility and the occasional chance you want a further +1 instead of striding/stepping away, if that's ever better for your caster/gish. The 3 for 3 is the most strange since as you mention no other effect has it, but is it weird because of that (which is a perceived problem, not necessarily a real problem) or because of internal math?
Anyway, for the 2 and 3 action version you need somatic and material components, which means it requires a free hand and has the manipulate trait, for whatever that brings such as Attacks of Opportunity.
Thus it isn't that easy to +3 and wield a 2H, in fact you can only do it while quickened and you can't even attack, only grip, which would be almost totally useless. That said, it's good you brought this to attention since Somatic doesn't require you to lose your grip AFAIK, so the 2-2 action could allow some abuse. A quick fix should be to ask for a M component from the 2-action upward so it'd make interacting with a 2H nearly useless, which I guess is the right thing to do balance-wise.
Finally, requiring some sort of tax, such as a feat or something in-between like a perk bonus (a la dungeon couch perks system) should be valid and less WILD.
Instead of messing with the AC bonus, which is tough to do in PF2's tight math, why not increase it's hardness when used to shield block? Each extra action could increase the hardness by 5, similar to heightening. (...)
That's also very valid and cool! My only problem with shield cantrip reaction is that it breaks it for 10 minutes and that hurts how I envision my fiction, which has mages and gishs flinging barriers during the whole fight. I get why they went that way, I just don't like it. I'll probably brew something along the lines of giving further shields castings during the same combat.