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.
Paladin once depleted the smites he has to long rest. Meanwhile fighter only need a short rest. Very useful in cases where long rests are being policed by developer. Paladin has alot of nice tricks unfortunately he has to use them carefully especially again when long rests are being used as a difficulty barrier by developers. It's being used in solasta, i'm quite sure it's possibly could be used by Larian as well. I much prefer rations where you can rest anywhere as opposed to "fixed" and "static" area where rests were intended by developers. It seems more like a "scripted" experience by developers to play the game it's mean to be played as dictated by developers.
I'm not sure if solasta is buggy or not. I have only once twin blade ever triggered as a reaction. I'm pretty confident that some attack rolls where my existing AC +3 would actually beat the enemy attack roll. Longsword on it's own is a 1d8 and lightning should provide another 1d8. so the calculation should be 1d8 + 1d8 + [STR modifier]. i have the ambidexterous feat and longswords are not finesse. as pointed, i didn't actually seems to notice that my off-hand proc the additional 1d8 frost damage. i do have the dual-wield feats so i believe my off-hand should add the STR modified as well. anyway with the damage comparing to pally i was to the point of depressed. maybe.. well maybe once i'm dual-wielding 2 longswords of dragonblade (1d8 + 2d6 Fire) and another Punisher (1d8 + 2d6 Piercing) on off hand that damage may actually compete with paladin?
IMO maybe rogue should get 2 times sneak attack per turn that probably would be much better. I'm really excited about multi-class i hope Larian really put it in. Now that's the only edge Larian has over Solasta at the moment.
Also... yesterday i just got counterspell... it was freaking amazing. I'm not sure if it's bugger.. but i can counterspell the enemy counterspell? woahh.. my melee was about to hit the enemy caster and he counterspell.. and i got a reaction to counter him!. that's pretty cool.