How broken is this ability

[Psyche]

I made the following warlock subclass, and I want to know how OP it is(especially the 20th level thing)
Spells: Disguise self, Charm Person
Alter self, Suggestion
Major Image, Enemies abound
Charm monster, Greater invisibility
Seeming, Dominate Person
Shapechange
Transformative: Beginning when you take this subclass, you can transform into a Celestial, elemental, undead, construct, abberation, fiend, or fey of CR 1/2 or less that does not have a swimming or flying speed as an action once per short or long rest, using the druid's wild shape rules.
Improved shapechanging: At 6th level, You can now transform into a creature from the above list of CR 1 or less, and you can forgo using the features of one or more of your warlock invocations until you finish a long rest to increase the maximum CR by one.
Shapeshifting savant: Beginning at 10th level, you can transform into a creature from the above list of CR 3 or less with a swimming speed, and you can now expend one or more warlock spell slots in order to transform into a creature with a max CR of +1 for each spell slot you expended.
Shapeshifting master: Beginning at 14th level, you can transform into a creature from the above list of CR 4 or less with a flying speed, in addition, at 20th level, you can expend all of your warlock spell slots, be unable to regain spell slots until you finish a long rest, use this feature, and forgo using all of your invocations until you finish a long rest in order to transform into a creature from the above list of CR 25 or less.

[JNAProductions]

FYI, we've got a Homebrew Subforum (https://forums.giantitp.com/forumdisplay.php?15-Homebrew-Design) for this stuff.

As for the actual question... Yeah, that's bonkers. At level 11, you can transform into a massive range of CR 9 see edit, CR 7 monsters once per short rest, while keeping two 5th level slots. CR 8 CR 6 if you need every single invocation you have.

A Hydroloth (CR 9) has at-willDarkness, Detect Magic, Dispel Magic, Invisibility (Self-Only), and Water Walk.
A Glabrezu (CR 9) has 157 HP, four save proficiencies (including Constitution), innate spellcasting, and the ability to cast a spell plus make two attacks at +9 to-hit, 10' reach, 2d10+5 damage and auto-grapple on a Medium or smaller target.
A Bhuer Hag (CR 7) has Hold Person at-will, and Control Weather once per day.

What I did with a similar idea, the Shapeshifter (https://forums.giantitp.com/showthread.php?646504-Shapeshifter-Mk-II-Shape-Harder), was make it so using abilities requires you to expend spell slots of your own. As you have it written, it's absolutely OP as all hell. Especially since I did NOT do a deep dive-I just looked at a couple of moderately high CR monsters.

Edit: For reference, a PC usually has a CR of anywhere from a third to two-thirds of their level. A CR 12 PC might have a CR as low as 4, or as high as 8. You could build a weaker PC, and you can probably build a stronger one, but typical PCs fall in that range. (That's not stated in the system anywhere, it's just observation.)

Even ignoring the CR 25 bit (which, I should remind you, one CR 25 monster is a DEADLY Encounter for four level 20 PCs), this ability is whack on the numbers alone. Not even accounting for being able to dumpster dive for the best possible form at the moment.

Edit: Realized I misread something, editing in a moment.

Edit: I misread it. I thought it was "+1 per SLOT LEVEL expended," not "+1 per SLOT expended."

So you can only go up to CR 7 at level 11. Which is still bonkers-I'm not seeing at-will Dispel Magic, but again, I only did a cursory search. It's still broken on numbers alone.

[Psyche]

Thank you! What would happen if I had it use 9th level arcanum to do this. So Instead of having it not use any invocations or spell slots, I have it use this ability to essentially give a +5 CR shapechange.

[JNAProductions]

Thank you! What would happen if I had it use 9th level arcanum to do this. So Instead of having it not use any invocations or spell slots, I have it use this ability to essentially give a +5 CR shapechange.

Shapechange is already a powerful 9th level spell. Increasing its power wouldn't be good, but ultimately, I'm less concerned about potential Tier Four shenanigans than I am with the lower ones.

[Greywander]

I'd start by making the 1st level feature something like at-will Alter Self.

At 6th level, Alter Self no longer requires concentration, and when you cast it you can disguise your aura to appear to be another creature type. I'd also have a short list of specific features cherrypicked from creatures of those types, allowing you to take on one such feature at a time.

At 10th level, maybe get access to a second such feature, and a slight combat boost (e.g. gain temp hp each time you cast Alter Self).

At 14th level, you can take on a third creature feature, and you have access to some stronger ones, including flight. You can also actually become a different creature type, not just appear to be one. Each type comes with it's own special traits, like undead not needing to eat, drink, breath, or sleep.

Something like that. Maybe using a spell slot would give you more features according to the slot level, instead of getting them as subclass features. This would free up those later features for something else.