Hi! First post and right off the bat I wanna share a build of mine. Some people don't like Theurges cause they take long to come online, but this build does what you need for the early parts of the story and by the time you start taking Sorcerer levels, your party will be well equipped to handle situations as you push to qualify as a Theurge.
Here we go!

Aasimaar Plumekith for Dex and Wisdom bonus.
Alignment: Any good (I don't care what kind of lore exists, an Aasimar should not be evil or neutral.)

Str Dumped
Dex 18.
Con 10
Int 12
Wis 20
Cha: whatever
Unless you're using this as your main char, you don't need charisma. You won't get good channel energy progression, so you don't really need it in that regard. You can leave it at 10 for your main char and put points in persuasion, you might fail a few checks, but nothing big... plus later you get the crown which gives +6 to all mental stats.

Take Religion and Arcana to qualify for the Theurge and put the rest in what you want.
Since this is a semi-support character with Wisdom as a main stat, I chose to focus on Perception.
Deity: Good Aligned Despite not getting the level 6 or 8 domain ability, you do get the domain spells even as you level as a Theurge.

Herald Caller gets summoning feats at levels 4 and 8. I wont go as high, but will pick up those feats, cause we still have spontaneous casting Summon Monsters, which will work on all the Cleric Spells gained through Mystic Theurge. I wont finish the build completely, leaving space for you to choose some of your own things.

Level 1: Herald Caller 1 - Spell Focus: Conjuration
Level 2: Herald Caller 2
Level 3: Herald Caller 3 - Augment Summoning
Level 4: Empyreal Sorcerer 1 - Superior Summoning
Level 5: Empyreal Sorcerer 2 - Combat casting (not to counter melees, but possible ranged attacks directed at you)
Level 6: Empyreal Sorcerer 3
Level 7: Empyreal Sorcerer 4 - Improved Initiative
Level 8: Mystic Theurge 1
Level 9: Mystic Theurge 2 - Choose a feat you like (my advice, Spell Focus and later Greater Spell Focus Evocation)
and so on, and so on.

Now for a wall of text explaining my build.

You're a character who has spontaneous Cure and spontaneous Summon Monster spells, you can cast these while stealthed and summon a group of cannonfodders right near the enemies. While they focus them you finish them off with your party. Granted, channel energy is very useful, but you can cast Mass Cures to make up for the lack of Channel Energy dice. Obviously consider taking metamagic feats, Quicken and Extend come to mind. Use the metamagic rods wisely also.

You can cast offensively and have an Okay-ish caster DC. Choose your spells wisely, choosing spells which will have the biggest chance of succeeding. Your typical strongarm fighter will have poor will and average reflexes, but good fortitude saves. Roguish and ranged characters have good reflexes, so don't cast any rays, arrows or fireballs at them... meanwhile they have poor will and low fortitude saves, making them perfect for fear/enchantment spells or gaseous attacks, diseases and the likes. Don't neglect your role as buffer though. Spells like Delay Poison Communal, Death Ward, Magical Vestment, Shield of Faith, Bless, Resist Energy Communal, Blur and especially Stoneskins and Displacement (cast Displacmeent with extend metamagic rod activated for tougher fights)

Why no point blank shot, precise shot? You can take it, but honestly...... if you're engaged in melee, you're doing it wrong. Even when forced into an unfortunate formation during cutscenes that leave you exposed, you are not in need of these feats. Keep Vanish and Invis potions on you, which you probably haven't been using much anyway. If you really want to pick up the ranged feats, go ahead, I'd personally add Spell Focus and Greater SF Evocation.

Never take uneven values for your main stat(s). If you're playing in a party and share exp to all your companions, you may not reach level 20, which is when you get your last stat increase. Had I chosen wisdom 19, this build would have 23 unbuffed wisdom at endgame, meaning it sucks and you'll miss out on 1 spell DC (and more spells in your book).

Go with a Light Crossbow as your weapon, later on an Artisan has a nice one which has an innate dimension door spell built in, in case you don't wanna waste a potion to escape.

You wont reach sorc level 9 spells, but you'll have kickass empowered summons at your disposal whenever you want. Lastly, Empyreal Sorcerer gets a lot of divine spells to his sorcerers spellbook, like bless, shield of faith, lesser restoration.... opening up spots in your cleric's spellslots for spells you'd normally not choose.

Well that's it... if you're reading this, I can't believe you read all of this. Thanks :)

Good luck and have fun.

By the way you don't really need spell penetration in this game it seems as there are few enemies with enough sr.

I tried to getting mystic theurge to work as a complete package and tried various builds for it. Your build has the summons spam going for it that is nice.

My personal favorite theurge now is glutton wiz/ erastil ecclesior glutton wiz/ asmodeus ecclesi. mixxed in with eldritch knight for the last few levels.

The reason fire domain gives you a bunch of ray fire spells so I try to go eldrich knight after theurge to salvage attack bonus somewhat.

I use nobleman's amusement bow to on this merc so I can take extra feats in spell focus conjuration or necro (in addition to evocation) as well as elemental focus fire_greater, and use spell specialization on sirrico or hellfire ray.
I use human race for bonus feat to take martial proficiency down the line to be able to go eldritch knight without sacrificing a feat.

End up with very high fire DCs (perticularly with certain items in the game) ...so really good sirrico and hellfire ray. (Ironically the nerf/ fix to Arcane trickster sneak on rays means for a ray caster going eldritch knight is better than AT anyways).

Final is 3 Ecclesitheurge/ 3 Thassolonian Glutton / 10 Mystic theurge / 4 Eldritch Knight....9th level spells are whatever for me because this is typically my secondary/ support caster with some dps through fire thrown in. Necro slots in glutton give boneshatter+animate dead, enervation, level 5 is heightened something, 7 is waves of exhaustion/umbral strike / 8 is wilting

Saving 2 feats on Point blank and precise shot feats lets me take as I said the schools of wizardry usually evo+conjuration or Evo+necro as well as the fire element feats.

I go Glutton as the toon is fragile and I appreciate being able to cast illusion spells like displacement and mirror image to not melt immediately.

I still use summons in terms of animate dead/ create undead but in general this is a CC+buff theurge, not much healing but I don't need it.

Hmmm that's good to know about the spell pen actually. Might choose improved initiative there then, combined with the good dex, you'll be able to vanish or go invis quickly if threatened by an enemy.