Are you playing vanilla or mods? Call of the Wild+favored class+proper flanking is recomended and adds a lot of useful stuff.
Firstly, don't level up the main character during the opening or after the fight with Oleg's (it's only two fights or so) until you can hire mercenaries. This will allow you to hire mercenaries for cheap immediately after the prologue (the cost scales with level since they join at your level). Even if you do use companions, having mercenaries with a variety of highest stats really helps during kingdom stuff since some advisor slots don't open till mid-game and don't have good options period, plus certain advisors will be temporarily unavialable due to plot at times.
The main character should be at least somewhat capable solo, as there's a few segments you're forced to do this. There's also some times you'll want to make a diplomacy check, so charisma or (with favored class for traits) intelligence based is preferable (though it's not really mandatory). Unlike NWN2OC or Wrath of the Righteous, the enemy types are relatively balanced (preference to Fey later in game) and nothing is just given immunity to everything to my knowledge so Save or Lose spells are really good so long as you don't focus sleep and paralyze (but those are good to pick up for early game where you primarily fight humanoids, animals and vermin). There's very few solo fights, so area of effect stats are prefered.
As for party members, any charismatic melee wants to grab Cornugon Smash as it's a great passive that makes save or lose from your spellcasters more likely to work.
As for non-build stuff: Don't sweat the time limits too hard. Just resolve the time limit stuff first then go back to adventurering while kingdom stuff plays out. There's no danger of running out of time unless you rest spam. One thing that really helps maximize your time between plot events is knowing that you'll be able to teleport between your villages mid-game, so place them with knowledge of that. Also never request anything from a kingdom artisan unless you know what you want, as telling them to do whatever you want boosts the chance of getting their special item.