Adding that 20th Sorc level to a Great Wyrm Solar Dragon

[Arkat]

How would you add that last Sorc level to a Great Wyrm Solar dragon?

According to Bestiary 4, a Great Wyrm Solar dragon casts as a 19th level Sorc. A Great Wyrm Solar dragon also has 29d12 HD (188.5 HP not counting CON, Toughness, etc.) and is a CR 21. The base saves for a Great Wyrm Solar dragon are Fort 16, Reflex 16, Will 16.

I'd like to add that last level of Sorc to effectively make the dragon a 20th level Sorc.

How would you do it?

Add a d6 HD or a D12 HD?

Would you add that extra +1 to Will saves as a 20th level Sorc gets that extra +1, or would you just leave its saves alone?

Obviously, you would add the extra 9th level spell and the extra castings per day, so no problem there.

I'm guessing the CR would also go up by one because that extra Sorc level would add +1 to the CR because Dragons are generally assumed to take the Caster role.

How about the BAB? A Sorc going from 19th level to 20th level gets a +1 to its BAB. Wouldn't the dragon also get a +1 to its BAB (for a +30 BAB total) if it gets that 20th Sorc level?

I'm not going to adjust its ability scores by +4, +4, +2, +2, 0, and -2 even though RAW, I could. Seems a bit OP for adding just 1 Sorc level.

Thanks.

[Azothath]

seems more like GM Advice. Exactly which spells is a matter of taste and experience along with thematic choices but RAW has a list under Ancient.
It gains casting ability, not class levels, class feats, and class HD... Mythic goes a bit bonkers and Wyrms are mythic.

Solar Dragon age categories
 Spells: A dragon knows and casts arcane spells as a sorcerer of the level indicated in its specific description. Its caster level depends on its age, as shown for each type.
 Spell-Like Abilities: A dragon's caster level for its spell-like abilities is equal to its total Hit Dice.
Solar Dragon, Ancient

Red Dragon, (Mythic) Wyrm

[Mysterious Stranger]

If it helps the way Hero Designer, does it is to add what a 1st level sorcerer gets. So, no increase on BAB, FORT or REF saves, but a +2 on Will saves and d6 extra for HP.

[happykj]

There is a drider NPC with cleric levels in Rival Guide.

Basically she get all the class abilties as normal and stack the caster level with her drider's spellcasting ability.

I would prefer to make it simple by adding the caster level only instead of adding everything.