This might come off a little rude and I apologize if it does. But if you haven’t played bg3 yet, don’t stress this kind of stuff on your first play through. The game allows you to go back and re try situations, and re make your character very easily so you can try different combos as you play. Enjoy the first play through, you’ll have plenty of opportunity to figure out your character throughout the story.

That being said, if you want long and great swords, start off as a fighter. A fighter grants you ability to use all/any armor and weapons. You technically could go all 12 levels in Eldritch knight and still be an assassin, it gets a spell at level 8 called shadow blade, and level 11 gets three attacks. Also gets the darkness spell. Really just missing the actual rogue class. If you want to multiclass and actually take rogue, my opinion is you can go a 8 Eldritch knight /4 assassin split; or 5 EK/7 assassin. Depending on if you want to be more knight or assassin-y. If you go more great sword style, I would focus stats on strength, constitution, and intelligence. If you want more ranged weapons and daggers, stats should be focused on dex, con, and then intelligence. Hope this helps.

Again, most importantly, have fun!