Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

If you only want to use Druid abilities in the UD, then Cavestalker 3.

If you want to invest in the Skills that Cavestalker increases via Level Ups, then crank up the Cavestalker Levels.

Your suggestion of Druid 15 / Cavestalker 10 / Thaum 4 is good. Druid/SS don't benefit from Thaum 5. The Cavestalker high AB will give you more attacks when you Wildshape or Shapechange.

Druid 16 is important to gain Elemental Shape.

You could also do Druid (12), Cavestalker (10), Fighter (4), Thaum (4), in order to gain Unarmed Attack bonuses from Fighter, and better AB.

If you really want your Companion to be the best, you'll need to invest in the Epic Animal Companion feat.

Gnome deities: Baervan Wildwanderer and Segojan Earthcaller get the animal domain for Animal Companion and Conjure Animals spell. You can see the other allowed domains here and check the specifics of the domains in here.

I would pick Segojan Earthcaller with Animal and Cavern/Earth domains.

The build I would go for would be something like Cleric 25/Thaumaturge 5, or something.