I generally enjoy pure BG1 (and really wish EE didn't erase the experience), but I also think BG1->BG2 conversions more than make up for the features lost in transition especially since you can't install SCS on vanilla BG1, and that mod really adds a lot of entertainment to fights and can surprise even veterans. And, as a bonus point, you can no longer cheese Drizzt with mass summon spam, so the game isn't necessarily easier or harder either way if you add some tweaks here and there.
Generally for BG1 though, I always had my entire party stay entirely ranged until around level 3-4, because bow and arrow (or sling) do not break like mundane melee weapons and the early combat is volatile enough to discourage going into melee (especially on Insane, where frontliners get one-shot in an unlucky roll of the dice), plus typical Entangle+Sleep+Web tactics synergize better with ranged weaponry. Don't think Mage/Thief would necessarily be any harder than a regular mage (who, for all his faults, has the I Win buttons in form of Sleep and Blind in the early game), although Thief levels by themselves don't really shine much in BG1, you're mostly a utility toolbox with some promising backstabs if you can find ways to raise your Strength really high (DUHM Bhaalpower) and Staff of Striking. Just stay at range a lot, you will pick up some colour around spell level 2 (Web, Horror). Generally though I believe early AD&D was balanced with the idea that fighters are the heroes of the low-level story and the wizards have to rely on their strong backs, so honestly, just keep everyone ranged and refine your kiting tactics (have someone act as bait). Also places like Gnoll's Keep are really good for ranged approach because the place is cramped full of chokepoints you can block with just a single meleer (though be very careful still).
Also, can anyone confirm whether BG1 or BG2 mechanics make your attacks from shadows/invisibility have a lower THAC0? Opening a fight with M/T from invisibility as a flanker with Chill Touch could be potent.
