"yes can you please fix polymorph spells so that they scale properly and Clarify rules on what can work with untamed form."
+1
I think it's very frustrating that Untamed Form is difficult to use.
At certain levels, you're locked into specific sizes.
You can't be anything but Large or bigger.
Adventure Path maps are cramped enough for Size to regularly matter, for example.
I think the Heightening of Form Spells should have the Size change be entirely optional, while still keeping the improved stats of Heightens. Just as a fundamental Quality of Life aspect. This is an issue with other spells too, like the Light Cantrip being unable to be shrunk back to 20/20.
Beyond that, you can't use Untamed Form for combat at level 1-2, or 19-20. So, 10% of the game makes it unusable for its clearly intended base purpose (combat, since it gives bonuses to that; and it requires Druid Feats to be effective for non-combat usage). It's especially absurd that the zenith of your power (LV 19-20) makes you rely on something that isn't what most of your Druid Feats are usually focused on (if you're focusing on Untamed Form in general).
It's entirely possible to invest >50% of your Druid Class Feats on improving Untamed Form, then get to level 19-20 and most of them just don't matter anymore since they're combat-oriented, but now you must use Nature Incarnate (a 10th-rank spell) to be good at combat in a battle form since any Untamed Form you take actually decreases your Caster AC. This comes from the form spells' AC scaling only within its "level band" but not beyond it. Level being added isn't enough.
It becomes optimal to retrain out of most of the Druid Feats that improve Untamed Form at level 19-20 because of this. I think that's a bad outcome of the design.
It's also a bit silly that you go from "I can enter this combat form as long as I have Focus Points." to "I can enter this combat form once-or-twice per day." since it shifts from relying on a Focus Spell to relying on a 10th-rank Spell Slot. It basically relegates you to not being an Untamed Order Druid for combat purposes.
It's also a bit silly that you "level out of" specific forms being useful. If I want to play a Druid who only shapes into Animals to fight, and not fantastical creatures like Phoenixes, Dragons, etc, I think that fantasy should be viable & supported. I'm happy we have Dragon Shape, and so on, but it shouldn't be required to switch to them to stay combat viable as the levels climb.
I think you should be able to take on any lower form and keep the stats of a higher form (so long as the spell - Untamed Form - is of the appropriate rank for the higher form). If I were to cast Untamed Form at level 18, and I have Dragon Shape or Monstrosity Shape, I should be able to become an Animal Form that has the stat improvements that Dragon Form or Monstrosity Form provides (primarily AC, but speeds & skills matter too).
Realistically, I think the best way to change it would be to give a status bonus to AC instead of recalculating it altogether, to upgrade the caster's AC accordingly (i.e. +2, akin to Untamed Form's bonus to melee). You could create a ceiling value with a formula to avoid shenanigans like a Monk getting a form spell cast on them, or you could lock this to Untamed Form. I think there are definitely options.