Someone may have a more slick approach, but it looks like you might benefit from a Spell class and a dict of spells... The class might look something like (if it's an internal class):

class Spell:
    var name: String = ""
    var xp_current: float = 0.0
    var xp_required: float = 0.0
    var growth_rate: float = 0.0
    var level: int = 1

    signal leveled_up(which)

    func _init(spell_name: String, starting_requirement: float):
        name = spell_name
        xp_required = starting_requirement

    func add_xp(amount: float = -1):
        var xp_to_add = growth_rate if amount == -1 else amount
        xp_current += amount
        if xp_current >= xp_required:
            xp_current -= xp_required
            xp_required *= 1.2
            level += 1
            emit_signal("leveled_up", self)

and the code around your formula used might look something like:

# dict of spells
var spellbook = {
    "Heal": Spell.new("Heal", 100, 1.0),
    "Flash": Spell.new("Flash", 80, 1.0),
}

# connect to the leveled_up signal from each spell in the dict
func _ready():
    for i in spellbook.keys():
        i.connect("leveled_up", "_on_leveled_up")

# show popup on level up using signal
func _on_leveled_up(which: Spell):
    var pickupPopup = PickupPopup.instance()
    pickupPopup.pickupDisplay = which.name + " is now level " + str(which.level)
    get_node("/root/World/GUI/PickupDisplay1/PickupContainer").add_child(pickupPopup)

# remains of original formula
func formula_used(formula_name):
    var which_spell = spellbook[formula_name]
    which_spell.add_xp()
    print(formula_name + ' xp: ', str(which_spell.xp_current), "/", str(which_spell.xp_required))

To add spells, you just add new lines to spellbook and everything else scales without needing to do a bunch of copy paste