Hello! I figured I should reach out to you since I don't think there's literally anyone else who can answer this question ahaaa.
but,
this is my current way to cast spells

apshop:
name: "&7Level Up! ($sp)"
description: Spend SP to Level up!
icon: totem_of_undying
inherit: false
hidden: false
actions:
cast:
- class: Selector
parameters:
confirm: true
auto_close: true
confirm_filler: stained_glass:13
placeholder_icon: gray_stained_glass_pane
show_unavailable: true
quiet: false
loud: true
limit: 100
cost_type: sp
title: "($balance&0)(AP-$ap)"
requirements:
- holding_wand: true
options:
- placeholder: true
- placeholder: true
- placeholder: true
- placeholder: true
- cast_spell: give_ap
cost: 50
cost_modifiers:
- scale: "x ^ 1.5"
type: attribute
attribute: skill_level
- placeholder: true
- placeholder: true
- placeholder: true
- placeholder: true

give_ap:
creator: MrSlushie96
creator_id: 67f4346qf-7203-4a3e-a439-ff92c2eda6c7
name: Level Up
description: Spend SP to level up!
icon: totem_of_undying{18001}
actions:
cast:
- class: CheckRequirements
requirements:
- attributes:
- attribute: skill_level
max: 100
fail:
- class: Cancel
spells:
- give_ap
actions:
- class: Command
command: mgive ap 1
op: true
- class: ModifyAttribute
attribute: skill_level
bypass_undo: true
value: "x+1"
parameters:
target: self
op: true

Basically players have to "buy" a level using the SP they have, and with that, they can go ahead and learn new spells based on their AP.
However, i'm wondering if the level up could be passive, with something along the lines of,

passive_level:
creator: MrSlushie96
creator_id: 67f4346f-7203-4a3e-a439-ff92c2eda6c7
name: Level up!
description: When you reach the right amount of SP,
you level up!
icon: scute{3}
passive: true
inherit: base_passive
toggle: none
toggleable: false
triggers:
- trigger: interval
interval: 1000
actions:
cast:
- class: ChangeContext
target_caster: true
actions:
- class: Cast
spell: give_ap
as_target: true
parameters:
target_self: true
target: self
costs:
sp: 50
cost_modifiers:
- scale: "x ^ 1.5"
type: attribute
attribute: skill_level

However, i don't think i got the cost_modifiers in the right position, or if i can even put it there in the first place.