So the Lycanthropy curse from Horror Realms makes it that you cannot speak during combat but it doesn't prevent spellcasting:
This ability works similarly to the tongues curse, but whenever you are in combat, you cannot speak at all. This does not interfere with spellcasting but does apply to spells that are language dependent.
The Lunar mystery's Form of the Beast revelation allows you to shapeshift as beast shape spells so it doesn't qualify for Nature Spell, and is therefore not great.
Would the Lycanthropy curse, which would be hugely thematic, allows you to cast spells while shifted?