Fighting styles are such minor buffs compared to Ability scores that you can't swap.
I am nto sure what you mean by this. Fighting styles can be swapped every time you get an ASI, but you still get the ASI. It is not either-or.
Basically te Ranger is the most campaign flexible warrior as it can go DEX and have bows/crossbows, rapier and Shield, and dual wield swords/axes backed up with hunter's mark, hail of thorns, fire arrow, elemental weapon*, ashardon's stride, searing smite, zephr smite, swift quiver, etc etc.
The ranger needs a feat to dual wield any D8 weapons and he needs strength to dual wield axes, which gets me to the biggest issue and that is being MAD.
A fighter can have a good (16+) strength and a good (16+) dexterity, while also having a 14 constitution and two more 10s. A Ranger is hard pressed to do that.
I don't think Ashardalon's stride is a Ranger spell.
Swift Quiver comes online at 17th level and uses a bonus action to allow 2 additional attacks on future turns using a bonus action for 4 total. The Ranger can do this in one battle a day with concentration. Meanwhile the fighter started making 3 attacks a turn, every turn as an action at 11th level with no concentration and no use of his bonus action and can do that all day long. With TWF or with the appropriate feats (which they have plenty of by that time) fighters can make a 4th attack with a BA every turn, all day long even with medium weapons. Finally fighters could add a concentration spell to this enabling not just many attacks but boosting damage as well, for example, unlike a Ranger; a Fighter with the right subclass CAN get Ashardalon's stride and can get it at 13th level or he can get Haste and make 4 attacks every turn (or 5 attacks with TWF) and at 17th level can do that in 3 fights a day to the rangers 1 and can do it with any weapon without loading, not just a bow. At 17th level he can also Nova on a turn and do up to 8 attacks (action, action, TWF, Haste) on a single turn twice per short rest.
As for the other spells, dual wielding is not going to keep up damagewise, both because you will be losing damage going with a d6 weapon (unless you take a feat) and because you will be using a lot of bonus actions to put up those spells and/or move them around.
Zephyr Strike is a good example. You cast it while TWF and you lose 1d6+S/D on that turn in order to gain 1d8 on a future attack ...... if concentration lasts until then ..... Hunters Mark is a little better damage wise in that you lose S/D bonus on the turn you cast it and then another S/D bonus on every time the target dies while you reassign it, to gain an extra 1d6 on your action attacks and 1d6 on bonus action attacks on other turns when you are not moving it.
If you actually do the math, if you are boosting damage with bonus action spells; TWF fighting style will generally provide less damage for a Ranger than dueling fighting style and a Rapier. Even if you don't want to use a shield in order to have a hand free for spells, a single Rapier is going to average more damage than dual wielding short swords if you will be using bonus actions for spell damage. If you are not using spells to boost damage that is a different story.
The final thing to consider here is concentration. All those spells you mention require concentration and in tier 1 and 2 that is not a big deal, but in tier 3 damage starts to become massive and Rangers are not proficient in concentration, so it is usually not going to last very long, especially if you are using your Bonus Action to use TWF or cast spells instead of using it to turn invisible or hide.
Certainly spells bring a lot of flexibility to the table, I am not arguing that, but as a pure martial Rangers are not as capable as fighters.