My party (5lvl+) already has:
Wizard with halfed "Disable Device" skill
Fighter with halfed "Lockpicking" skill
Cleric with "Find traps" spell
Monk with full hiding skills and ability to multiclass over Thief

But Monk class known as a very good fighter, and Thief known as a very mediocre one
I am wary to lose the "momentum" with my Monk

Do I really need to take the Thief levels or my party gonna be fine?

This is a really bad idea and very unnecessary. Just make for example this character:

- Human race for an extra skillpoint per level
- Intelligence 14 for two extra skillpoints per level
- Otherwise fighter stats (high Strength and Constitution)
- Start as Rogue for maximum skillpoints
- Continue exclusively as Fighter afterwards

Human does not get xp penalty for the class that has the highest level, in this case Fighter, so this character will never receive any xp penalty.

Thanks to Int 14, Human race, and starting as Rogue you have plenty of skillpoints to distribute, even on Fighter levels. Enough to cover all important Rogue skills.

Thanks to being one level of Rogue, all thievery skills can be leveled as class skills, and thus maxed, like on a pure Rogue.

Thanks to all other levels but the first being Fighter you have a fully valid Fighter character otherwise, with very quickly almost no disadvantage compared to a pure Fighter.

Other non-spellcaster classes of course would work just as well as Fighter; Fighter is just such a good class in D&D3, as they are really in every version of D&D.

You can also make a sacrifice and do the same with a spellcaster class like Wizard, but that seems totally unnecessary.

All that said, I have played IWD2 with a single Drow Sorcerer, and it was perfectly possible. You can handle traps and locks without a Rogue just fine.

WARNING: I havent played IWD2 in over a decade and I cant find any good IWD2 Wiki to confirm the details. Furthermore I only own IWD2 on DVD, not as GoG game, yet, and I have no computer with a DVD drive anymore. So if I made an error I'm very sorry; I tried my best. This is definitely a valid D&D3 character but IWD2 ran on an AD&D engine (Bioware Infinity) only very losely converted to D&D3, which for example lacked Attacks of Opportunity, so it was not always accurate in its D&D3 implementation.

Still according to the original rules of D&D3, just taking one level of a class is enough that the skills of that class are your class skills afterwards, even if you dont level that class on that levelup. I hope thats the case in IWD2, too, though I have no way to make sure.

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TL;DR: Pick one character with sufficient skillpoints, give them ONE level of Rogue (preferably the first when you get additional skillpoints), and they should easily be able to handle all locks and traps.

Thanks for answer Geronimo!

I've been testing it out.

Clarifying my first post:
- I spent 2 hollidays to get my party 5-6 lvl by reading all dialogues and completing all sidequests with only checking the guide twice. Also with Widescreen mod game runs very unsmooth, it is hard to aim the cursor on 30fps. Therefore it would be very unfun to start over, so creating a proper character is not an option. It is my "first" WT, last time I played it was like 20 years ago.

- Also the question is - am I going to be fine without "Disable Device" and "Lockpick":

"Disable Device"
In IWD1 I was quite fine by tanking all traps, but the Yxonomei battle was very hards because of traps.
In IWD2 I don't really noticing the traps (I've just entered the ice palace level). In both cases we have "Find traps" spell to avoid them;

"Lockpick"
In IWD1 we have the Knock spell (for better experience it is better avoid memoizing Knock and Identify).
But the Bash-Strenght dependensy seems very strict.

In IWD2 I can't find Knock spell in manuals, this is great.
But the Bash seem to be vague in some boundaries. So I just can spam Bash to open a lock. Plus seems there are more ways to buff STR, and they are cumulative. With the Rage only my Barbarian has 27 STR, so I hope to open most of locks with that. But ofc I like to pick the locks more.


About your answer and gamerules:
- I've tried to take one thief class with my Barbarian and return to the Barbarian at the next level up. Unfortuantely having one thief class doesn't allow to buy skills for 1 point with your main class;

- I also hoped to simply buy some skills for the price of 2 points, but they’re capped at 4 ranks, making it mostly pointless. The exception is 4 ranks in Concentration for a nice Fighter perk;

- I still have an option to multiclass my mage into Thief. She has high INT, but sacrificing the mage levels would not be fun :|

- Yes indeed it is not true D&D3, but I love it and things like no "Attacks of Opportunity". It really feels like
extension of IWD1 with fixed and different engagements (more mages. at least).

- Next time I will take thief class for sure or some of its kit. Without multiclassing it maybe not that strong, but I dont play on Hard as well :)

I'm sad to hear that it doesnt work in IWD2 as it should, according to the original rules.

It definitely works like this for example in NWN2.

Update:

we have the Knock spell I've met a spell trader with it when my Sorcer can cast 6lvl arcanse spells.

I googled manuals and Knock was omitting there, hence I thouth developers got rid of this crude spell.
But we have it and my situation is mostly saved. I left to miss only a top rank ability to disable device, but traps yet were nearly harmless, i.e. traps are much weaker than in IWD1.

Yet answer to my question looks like : you dont need thief if you are trying hard and exploiting.

A bit late, but still try to answer.

You don't need thief. Sure, multiclassed wizard with one or two rogue levels works fine, but you don't even need that. Most locks can be bashed or opened magically. IIRC there're two or three deadly traps in the game, so you can tank most of them.

Well yes, but I eventually regret by not having a Thief.
I have just passed Dragon's Eye now.

I can't replace Search with divine Find Traps. My monk with 6 Search perk has more success in finding traps in Dragon's Eye than Cleric.

So I'd rather had a better quality game with Thief, who can search, untrap, lockpick than having a Monk.

Also Thief is very nice archer, and there are alot of good ranged weapons to utilize.