| Welcome to Gamebanshee.
This probably isn't what you were expecting, but my suggestion is to keep Jan and make Imoen your fourth party member. The Ascension Mod has a special role for her which makes things interesting, and it's always nice to have Jan and Minsc together for the entire duration of the game. Since you obviously don't mind modding the game, you might consider editing Imoen to make her a more effective character. If you do that, I suggest turning her into a Sorcerer. Either way, the idea is that you should play her as a Mage. There's nothing wrong with having two Mages (Jan and Imoen) in your party; in fact, I prefer to have as many spellcasters as I can.
There are two alternatives. The first alternative is to recruit a different character who is an even more powerful Mage. Edwin is such a character (Nalia isn't any more powerful than Imoen, and Aerie is in a different class), but Edwin doesn't get along with Minsc so you should take that into account. You might also consider an NPC mod such as Kelsey, Tashia, or Tsujatha, to name a few off the top of my head.
The second alternative is to add a powerful melee character to your party instead of a spellcaster. I'm not familiar with all of the battles in Tactics (personally, I think most of them are contrived and simply ridiculous), but I have found that melee power is very effective and sometimes indispensible in TOB, especially in the final series of battles when Ascension is installed. If you want another melee character, you can't go wrong with Keldorn, Korgan, or (later in the game) Sarevok. It's debatable which of those characters is the most powerful, but my vote goes to Keldorn. (There's an old message thread in this forum in which we debated the issue.) You might also consider Anomen, who is almost as powerful as one of my favorite characters, the Berserker dualled to Cleric. Granted, he is annoying, but in a one-on-one battle with sufficient planning and preparation, he could match or beat all of the other NPCs.  | 
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