So is straight sword and shield good in end game? Or should I eventually move to mystic knight?
And I know min/maxing isn't necessary at all, but is mystic knight still worth it if you do all fighter-ish classes early game?
Well game is defined by classes, not weapons. Specifically, each class can only use certain weapons and conversely certain weapons can only be used by certain classes. So sword and shield is basically Fighter, and also Assassin. Mystic Knight uses sword and shield too, however they use magick shields which are a special subcategory of shields that do cool stuff when you perfect block with them but have less physical stats.
Both are fine. Every class is, there's no real bad class. Warrior and Mage potentially I'd put at the bottom, unless Warrior can be modded to have 6skills. Mage because it's really boring and not very strong and you're pretty much always better off using a mage pawn instead.
Fighter's good all the way through. You can use the taunt stuff to make sure the enemies stay grouped around you for your sorcerer to aoe, and damage wise it's very good, much more than what you'd expect from a sword n board char. Sword skills have good reach, good aoe and are quick to execute(and you can use one to slash stuff in the air, great for harpies and other annoying crap) while shield skills are great for defense obviously but also controlling stuff.
Mystic Knight plays a lot differently, although you can play one like a Fighter, only with elemental buffs of your own. But it has a variety of spells, like creating areas that inflict damage over time to enemies and creating magic canon that fire projectiles imbued with your element when you hit them.
You'll probably want to play both at some point to get the augments iirc. Shield block stuff specifically. So at that point you can see which you prefer, there's no real penalty for switching.
And if you want a more agressive approach, you can play sword and shield assassin. Not as good defensively or control wise, but a lot more damage.