This is a tough one!
I agree that Spell Focus can be pretty useless for some spell schools, but making it universal might be a little OP (that’s a flat +1 to DCs, essentially). If I were to homebrew something, I’d probably have Spell Focus grant some sort of minor boon in addition to the DC boost. Think along the lines of Varisian Tattoo and how it grants minor spell-like abilities: https://aonprd.com/FeatDisplay.aspx?ItemName=Varisian%20Tattoo
I can’t open the PDF at the moment, but I’m assuming it reads as arcane-only because: a) that is how it is in the original book, or b) it’s a typo. I’d default to whatever the Core Rulebook says as we didn’t intend to change anything with Spell Focus.
Hope this is useful!