🎀 Wisteria 🎀

I would like to combine science with some kind of combat skill, so that I can be useful everywhere.

If, as in Rheims, it is possible to have a separate leader and a separate tactician (I need Revan's answer), then there is leadership. As an option, take some school of magic.

We should have all leveled up assassins and completed quests, destroying enemies before they even engage us in battle.

An assassin can defeat an entire group of enemies in one turn.
By the way, I planned to later teach my character invisibility, so that I myself would have normal stealth, and also be able to protect various crafters and wounded characters from harm. But that will most likely be after taking denial and some enchanting skills.