I'm not being conservative so much as realistic, given the situation. (
tarkin was conservative.) A group where just about anything goes can be rather lovely - I participated in one in a PbP game, and I'm working on an entry for a second
here.* (Believe me, I can push at the rules just as much as you.
) But you're still dealing with a DM of some sort. Dismissing a concern as "DM intervention" ignores the fact that one exists. And most games aren't quite that liberal; any live DM I've played with (not many, to be sure, fewer than the PbP ones) would balk at some of those ideas. So you hope for the best, but plan for the worst. (Note to OP: this means you'll want to ask beforehand. If the DM's fine with Reserves of Strength, by all means ditch Enhance Spell.)
(Plus, I'm not the first person to bring forth that particular argument against Fell Drain magic missiles; it was one of the minor proverbs of 339.)
Come to think of it, OP, what sorts of enemies might you be facing? (For instance, Manifest Ethos is far better if you're facing enemies from only one side of the alignment spectrum than when you're facing everything, or even just non-good stuff.)
Following Kell's UM suggestion, if you take Spellthief, also grab Master Spellthief. It mildly complicates things, but will give you a rather powerful CL. Also, come to think of it, has anyone actually made a guide on constructing epic spells - especially under certain common houserules like maximum unmitigated DCs or limits on mitigation? Because I occasionally see comments along the lines of "make epic spells, go to town", and some people may still be scratching their heads.**
Come to think of it, do you have a way of greatly increasing your save DCs? Because if you don't, your opponents will reliably make their saving throws. This is why single-target effects or Spellwarp Sniper are useful - they change the (laughable) saving throws to (not so laughable) ranged touch attacks or outright auto-hits.
*Even there, epic spells are capped somewhat, so I was having some difficulty trying to determine a way to reliably capture Soneillon (Champions of Ruin). Until I found Greater Planar Binding. And Superior Planar Summoning.**Rough tips: Go for buffs. Ignore anything involving offensive saving throws unless you can crank out high DCs. And even then be wary. Summoning sucks unless you're grabbing something unique (which Gate can't do well). Wards against common spells, especially if you can ignore them yourself, are good. As are Mythals. And the Shadow seed (which can make other creatures). Followers are incredible at mitigation if you can get enough. Never pay the crafting XP if you can afford to use a Rod of Excellent Magic instead.