if that's the case fighter is far superior than other martial classes. even paladin wouldn't be able to match fighter damage at higher levels? also action surge is per short rest. in dnd5e it seems like there's no limit to how many short rest you can take. dual-wielding on the other hand seems really bad to me. was wondering if the off-hand weapon procs elemental damage? it seems in solasta the off-hand weapon doesn't proc elemental damage. correct me if i'm wrong? if it procs elemental damage then dual-wielding maybe better than two-handed weapon. my ranger in solasta currently is dual-wielding 2 longswords one with 1d8 lightning and the off hand is 1d8 cold damage. however i don't seem to see the cold damage being proc. i have previously tried a rogue for few levels and then i abandon the party. rogue seems very bad at damage compared to my pally that burst down bosses fairly quickly.
also that makes multi-classing with fighter would be really strong. just 5 levels in fighter with action surge could net 4 attacks per turn excluding haste. not sure follow-up strike is a solasta thing does bg3 has it?
For pure basic attacks, sure fighters get more --> more total damage. But other classes (e.g., Paladin) have powerful abilities (e.g., Smite) which can make up for the fewer attacks. Action Surge is essentially 1x per combat, but a Paladin can Smite as many times as they have spell slots, a rogue can sneak attack every turn, etc.
If the weapon itself deals dice of elemental damage, then the offhand attack should deal that damage. The only restriction for off-hand attack damage is that it doesn't deal your modifier in additional damage. So your ranger's MH should do 1d8+Dex (or Str) lightning damage, and the off-hand should deal 1d8 cold damage. You have to specifically use your bonus action to attack with your off-hand weapon though; it doesn't happen automatically like in BG3. And you presumably have some feat or ability to dual-wield long swords..??
Rogue is great if you constantly get sneak attack damage on your turn AND if you also make frequent opportunity attacks (which deal sneak attack damage); otherwise yeah the damage is only okay.
"Just 5 levels in fighter" ... 5 levels is a lot, especially since most campaigns don't go above level ~10. Most people take 2 levels in fighter for the Action Surge, and rely on their base class for either Extra Attack or other features that benefit from action surge.