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Beautiful setting. But many reviews tell about the grind. So how should i develop my charakters and what other strategies are there to consider for a more fluid playthrough?

The game is not grindy in the classical "you need to fight mobs until you are strong enough for the boss" way.
There is a fixed amount of enemies, that are placed in the world and they don't respawn (except for the ones on the world map). The grindy part is that you have to kill all the enemies in a dungeon, so they will stop bothering you.

When you level up, you only get more life points, spell points (magic user only), training point and occasionally the amount of hits, the character does with a normal attack is raised. You don't really get stronger by leveling up.
Attack and defense are completely equipment based. Try to find hidden treasures, which often contain magic weapons, to get stronger.

Your training points can be spent on four skills at a trainer.
Close combat raises you hit chance with close range weapons like swords. Raise this one fast for Tom and Drirr, to make the battles less tedious.
Ranged combat is the same, but for ranged weapons.
Critical hits is super expensive, but a critical hit will usually kill an enemy with one hit.
Lock picking is not very useful. By the time you finally find a trainer, you should have enough lock picking tools, which have a 100% success chance.