Why the HELL isn't there an aberration shape I-IV spell series?
Okay, polymorphing is stupidly complicated (and the last time I was a druid was 3.5), so there may indeed be a huge logic hole here. Does this work? If so, here's the key feature to the Radiation bloodline, our newest complex!

Aberrant Shape (Su): At 3rd level, you gain the ability to turn yourself into a specific Medium aberration, a chaos spawn, and back once per day. This ability is treated as though it were a spell of the polymorph subschool, except as noted here. The effect lasts for 1 hour per sorcerer level, or until you change back. Changing form (to chaos spawn and back) is a standard action and doesn’t provoke attacks of opportunity.

A chaos spawn is a very specific type of aberration that has Hit Dice equal to your sorcerer level + half your levels in other classes, rounded down, and natural armor bonus to AC equal to your Charisma modifier. Like polymorph abilities that turn you into an animal, dragon, plant, vermin, or so on, all of your gear melds into your body in accordance with the rules for spells of the polymorph subschool. While transformed into a chaos spawn, you gain darkvision 60 ft. and proficiency with all natural weapons granted by your new form. You lose your ability to speak while in chaos spawn form because you are limited to the vocal apparatus of your new form, but you can communicate normally with all other aberrations while in such a form.

When you transform into a chaos spawn, select a mutation gained in place of a bonus spell. For the duration of your transformation, you have the selected mutation. At 5th level and every five levels thereafter, select an additional mutation to express while transformed into a chaos spawn, to a maximum of five mutations at 20th level.

You can use this ability an additional time per day at 7th level and every four levels thereafter, for a total of five times at 19th level.

At 3rd level, select two mutations that are gained in place of bonus spells for which you meet all the prerequisites. You gain these mutations.