Thief build for Baldur's Gate (EE)

I'd like to roleplay an actual thieving character through BG, as in someone who breaks into houses, picks pockets, steals things, whether through skill or con artistry.

What is an actual good build for a thief character? I'm finding that Pick Locks skill works well early on, because there are plenty of difficult locks early in the game but nothing in NPC pockets. And I know there are some very powerful items that can be obtained via pickpocketing in the middle of the game.

I know that thief/illusionists are also great mid game, especially for the backstabbing/illusionist combo.

But what is a good build otherwise? I hear a lot of guides recommend thief/fighters, but that seems to be more geared towards assassin types, rather than 'pure' thieves. The extra XP cost of fighters also seems to limit it.

Honestly, there's very little reason to play a pure thief in BG 1. Between the numerous thief party members available (it's the most common NPC class by an order of magnitude), and the relatively low number of points needed in most thieving skills to be effective (even further reduced by the ready availability of thieving potions), you just don't need more than 8 or 9 levels in the class. BG2 made up for this by introducing High Level Abilities, from which Thieves draw the best pool by far, but this isn't a factor in the BG1 EE (at least not yet), so I wouldn't build a character with an eye towards BG2.

That said, you have a few strong options:

In general, you're better off dual classing into thief rather than out of it. A Kensai dual classed into Thief can be a very strong melee combatant, for example. If you do intend to start as a thief at level one, then using the Swashbuckler kit is a very good idea - you give up the Sneak Attack ability but the +1 AC, hit, and Damage /5 levels, plus access to weapon specialization and full two weapon fighting means that Swashbucklers can hold their own in a toe to toe fight. More importantly, Swashbucklers have a full compliment of thief skill points, so even if you're only starting with 5 levels before a swap, you don't really lose much.

The other very viable option is to go full into traps, possibly (but not necessarily) using the Bounty Hunter kit. If Baldur's Gate is a game built around the identification and exploitation of cheesy tactics, traps are, by far the cheesiest. They're also wildly effective - if you invest in the skill. They do solid damage, from a BH they'll offer a variety of useful debuffs, and they're relatively easy to manage.

And finally, as you note, there is always the classic Elf F/M/T option, which will be able to do literally everything. The slower leveling is just not a sufficient hindrance.