Embiggening monster for higher CR for Polymorph [Homebrew]

Although the party wizard has had a wand of polymorph since finding it in a hand-converted U3 (before the release of Ghosts of Saltmarsh) she has typically used it defensively, turning enemies into slugs and such. Only recently has she begun to use it offensively. After fighting some carnivorous apes, she turned the party member L7 Barbarian into a CR7 Giant Ape when he was low on hp.

Polymorph

The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn’t have a challenge rating).

As a result of that battle, the party has leveled, and the wizard can now polymorph the barbarian into a beast of CR8.

Unfortunately, I know of only one CR8 creature that is a beast – the Tyrannosaurus rex. While RAW there is no restriction on using polymorph to achieve forms not seen before, it would harm the verisimilitude of my game, in which none of the characters have even heard of dinosaurs.

However, if the wizard knows that Giant Apes exist, I think it is plausible for her to reason that there must be even-more-giant Apes, and simply try to make the barbarian bigger the next time she casts the spell.

Thus, I am considering allowing a larger version of a Giant Ape, in which some of the stats have been embiggened to achieve the low end of CR8. I am interested in responses to this as a homebrew concept.

Useful answers could include:

  1. Information about other publicly-available non-dinosaur CR8 beasts
  2. Conformation or refutation of my assumption that such ‘variable size’ polymorphing is not allowed RAW
  3. Conformation or refutation of my assumption that so long as the new form is a variation of an official one and the CR is calculated correctly, there are unlikely to be balance issues.
  4. Confirmation or refutation that I have calculated CR correctly for the larger version (below).

CR7 Giant Ape
AC12, 157hp (15d12+60), At. 2 @ +9 to hit, 3d10+6 dmg, ADPRH 45

CR8 Proposed Giant Ape [all other stats as RAW Giant Ape]
AC13, 168hp (16d12+64), At. 2 @ +9 to hit, 3d12+6 dmg, ADPRH 51

CR7 Giant Ape Calculation
Hp 146-160 = CR6, AC is more than two below 15, adjust DCR -1 to 5
DPR 45-50 = CR7, to hit is more than two above +6, adjust OCR +1 to 8
Average CR 6.5, round to CR7

CR8 Proposed Giant Ape Calculation
Hp 161-175 = CR7, AC is two below 15, adjust DCR -1 to 6
DPR 51-56 = CR8, to hit is more than two above +6, adjust OCR +1 to 9
Average CR 7.5, round to CR8