Xeviat
Dungeon Mistress, she/her
1. The fighter has no damage dealing advantage over the other martial classes ...
Action surge, expanded crit, superiority dice, and 3rd and 4th attacks say otherwise.
2. Strength based PCs suck at range being reduced to throwing javelins often at disadvantage. As a further kick in the balls they can only make 1 attack a round since you can only draw 1 weapon a round.
Yes, str lacks range. You should be able to draw and throw multiple javelins as drawing ammo is part of firing it. Maybe I'm wrong, but that's how I've understood ranged attacks.
3. Dex based melee tends to be underpowered along with dual wielding which uses the bonus action. That bonus action is often better off used for Polearm Mastery, Great Weapon Fighting, or moving around hunter mark/hex or casting quickened hastes or whatever.
Those other options require spells or feats, so they should be better. Dex melee is fine with duelist on fighter; rogue and ranger are better for twfing.
4. Most classes casting spells.
Yes.
5. Other classes tend to be better targets for haste/twinned haste. THis is due to spells like hex/hunters mark and advantage to hit that Barbarians and Avenger Paladins can easily get.
Classes are different. Different buffs are more optimal on different classes. I have found that only a gwfing paladin or a twfing rogue out damage a gwfing fighter, and that's even the champion and requiring the paladin to use all of their spells for smite. The fighter has second wind every short rest, giving them amazing recovery in long days. Indomitable is huge.
All the fighter needs is liberal application of skills, since they lack hard coded spells.
They may be mundane, which isn't fun for some people. Try the Eldritch Knight.