Help me design a character with a magical girl transformation.

ok, basically: character wants magical power. He takes himself super seriously, is a bit of a misogynist, and is just kind of an entitled jerk, thinking that he DESERVES magic. So he summons a fey to attempt to make a deal and get such a power. Chesiree cat style Patron offers him the power. literally for free. but to use it, he must turn into what is essentially a Magical girl. Outside of this form, is functionally a rogue. but in the other form, becomes a powerful fey warlock. can only use this form for a limited amount of time, and he has to take an entire turn to activate this from (bonus action, action, and reaction. the transformation sequence is long.) but gives powerful buffs. How should I go about this? i want the power gap between the two forms to kinda balance it out, as the non magic form is significantly weaker in most regards. I'd love to hear you guys' advice.

The way I would do this:

1) The base character's stats are highly skewed towards Warlock vs Rogue -> i.e. their highest stat is Charisma not Dex, they are proficient in Charisma skills not Stealth, and they take feats suited to Warlock not Rogue. 

Assuming 2024 rules, make a custom Background: Noble Dickwad: STR/INT/CHA, Skills: Deception, Intimidation, Background Feat: Lucky

ASIs (Point Buy): STR 12+1, DEX 14, CON 14, INT 8, WIS 8, CHA 15+2

2) Build them as a Rogue since all passive abilities should reflect their rogue-ness not their warlock-form, but make yourself a homebrew Rogue subclass for the warlock-form.

Class: Rogue, Skills: Acrobatics, Athletics, Persuasion, Sleight of Hand, Expertise: Deception & Intimidation

Rogue Subclass : Magic Girl.

3rd Level : Magic Pixie Transformation!

At the start of your turn you can shout out your catch-phrase and transform into a magical version of yourself known as your Magic Pixie Form. Once you have transformed you are incapacitated and your Speed is reduced to 0 until the start of your next turn while you go through the intricate animation to transform. While in this form you gain the following benefits:

• You MUST use your Charisma for attack and damage rolls using any weapons you are holding instead of Strength or Dexterity.
• Your weapons can deal Radiant, Necrotic or Force damage instead of their regular damage type.
• You can use your Cunning Action to cast Misty Step at will. In addition, when you reach 5th level, you can cast Misty Step whenever you use your Uncanny Dodge.
• You can use your Action to cast Faerie Fire, Phantasmal Force or Sleep at their lowest level at will, you use your Charisma as your spellcasting ability for these spells.

At 5th level, while you are in your Magic Pixie Form you gain an additional 2 options for your Cunning Strike feature:

Taunt (Cost: 1d6). You taunt and distract the target, they must make a Wisdom saving throw against your spellcasting DC (Charisma). On a failed save, the target has disadvantage on attack rolls against creatures other than you until the start of your next turn.

Vanish (Cost: 2d6). You turn Invisible for up to 1 minute. This invisibility ends if you make an attack, deal damage or cast a spell.

This transformation lasts for 1 minute, you can use it a number of times equal to your Charisma bonus, you regain all uses when you finish a Long Rest.

9th Level : Manic Dream Step!

Whenever you cast Misty Step while in your Magic Pixie Form you can apply one of the following effects:

Nightmare Step. Choose one creature within 5ft of the space you disappeared from or appeared in, that creature must make a Wisdom saving throw against your spellcasting DC (Charisma), on a failure they take 2d10 psychic damage.

Blissful Step. Choose you or one creature within 10ft of the space you disappeared from or appeared in, that creature gains 2d10 temporary hit points.

13th Level: Magic, Magic, and more Magic!

You learn 5 spells of your choice from the Warlock spell list of 5th level or lower. When you activate your Magic Pixie Form you can cast one of these spells as part of that action (concentration on this spell is not broken by the incapacitated condition imposed by that action). Charisma is your spellcasting ability modifier for these spells.

17th Level: Whoop! Whoop!

You are immune to Charm and Fear effects, when you are targeted by a Charm or Fear effect you can use your reaction to Misty Step then make one weapon attack against the creature that attempted to charm or fear you if they are within your reach / range at the end of your Misty Step.