
Errenor |
But since there are only very few abilities that don't have the standard "the DC equals your spell DC or class DC whatever is higher"
Yes, only class abilities never have this, they use only class DCs.
Of course as far as I see no caster classes use their class DC (apart from warpriests who can use spell DC instead). And for non-caster classes class DC abilities aren't frequent, but which do use it are often rather important. Also weapon specializations use it.
SuperParkourio |

It's important for my character because I use a warhammer (Dwarven Weapon Familiarity) with hand of the apprentice. Crit specialization knocks prone on a failed Fort save vs class DC. At later levels, this is going to start falling behind, so I'm thinking of transferring all my runes to a pick now before it gets more expensive down the road. But then I saw "Class DC 25" on my character sheet and I wondered if I was missing an errata.

Easl |
Out of curiosity, how often do you expect your Wizard to crit with the warhammer?
Even if it's just on a 20*, chances are above average that he won't go more than 14 rolls in a row without critting. At level 7 he's probably got 3 focus points, so if he's casting Hand 3x per combat and getting his focus points back between combats, that's still something that happens every five combat scenes or so. So he probably sees it happen maybe once a session? Which is definitely often enough to want to understand the DCs of the checks it creates.
(*Obviously, if his Spell attack roll makes crits on 19 or lower, that expected rate goes up really fast. 19+ makes it likely you'll get one in the next seven rolls, for instance. And this seems like the sort of spell you might wait and strategically deploy after debuffs have given the party a higher chance to hit. OTOH, if he's casting it less than 3x per combat, that expected rate goes down)

Eoran |

I had not considered Hand of the Apprentice. That makes the frequency marginally higher than I was estimating.
At highest levels, the difference between trained in class DC and legendary in spellcasting DC is a 30% difference in success rates. Which is noticeable.
Personally, I would not adjust my own combat style even for a noticeable success rate increase for something that only happens once in five combats if I spend all of my combat resources on that one tactic.
In any case, the rules themselves seem clear enough. Individuals, including the Pathbuilder app developer, can houserule things as they see fit.

SuperParkourio |

At some point, I was thinking of taking the Advanced School Spell feat to get a second Focus Point, but I don't see myself using Interdisciplinary Incantation much. Spending a Focus Point to get the ability to cast a spell I see would be pretty neat if I didn't still have to spend a spell slot to do so, let alone one of the same rank.
On top of that, the spell needs to come from another arcane caster. I can't count on running into arcane caster enemies, and in PFS, I can't even count on allying with other arcane casters. Though I imagine the latter could lead to some hijinks.
Arcane Caster Ally: Casts lighting bolt and discovers weakness to lightning.
Wizard: Prepared a different spell but sure, why not?
If the trigger was the casting of a spell on the arcane spell list, I'd reconsider, but the triggering spell needs to literally have the arcane trait, as when cast by an arcane caster.

Easl |
So it's difficult to assess how much this will actually matter to you without more detailed information about when and how you use the spell. But even 2x per combat, if your crit chance is 19+, is sufficient that this situation will likely occur once a session or so.
But I think you've already got your answer, yes? The class DC is what it is, and when considering whether this is a good cast for you, you have to factor in that your class DC will not get high. Don't take that as a negative, it could still be a cool build. It's just to say there's not much you can do in terms of build and feat choice to change it.