What Difficulty to Go With?
It depends enitely on whether you want to use your BG3 experience to teach yourself how to play D&D properly. If you do you should play on balanced. If you don’t care about that, or maybe don’t have enough time etc, then play on Explorer and enjoy the story.
If you start on Balanced you will begin getting your arse kicked. You will then have to use resources (such as watching good players on Youtube and asking for advice on how to beat battles here) to work out why you are getting your arse kicked. This will take time and effort.
But if you stick at it you will find it starts to make more sense and you will be able to begin working out how to approach tricky situations yourself, how to use your characters better.
At that point you will realise that the effort was worth it. The experience you’ll have will surpass that of explorer because you will have achieved something difficult at the same time as enjoying the same story ride and the whole adventure will be much more exiting because of the much greater danger.
That’s one reason why I recommend you start on Balanced and stick with it provided you’ve got the time and the inclination to put in the effort and face up the game initially laughing in your face as you hit quickload for what seems like the 100th time.
The other reason is that there are several other games very similar to BG3 that are also very good, not quite as good as this one, but very good none the less. Examples are the Pathfinder and the Pillars of Eternity games. These games are actually bit harder than BG3, particularly the Pathfinder ones, and you will be able to enjoy them far more if you’ve learned to play BG3 on Balanced already, the skills are directly transferable.
Which ever route you take, have a great game and good luck!
While you are learning the mechanics I’d go with explorer. I’d also choose a fighter, a monk, a rogue or a barbarian as your first class. They have less to keep track of than the casters. You’ll get a couple of casters in your party and learning to use them effectively as party members is preferable to learning how to do it from scratch and not know what you are doing.
Gale and Shadowheart are pure casters (wizard and cleric) and Wyll is a Warlock so he’s kind of a easier caster to work with as he has less spells and spell slots to worry about, and you get them back on a short rest with him compared to the long rest with Gale or Shadowheart.