It's not just about that Land, Sea & Air, but also Challenges they Represent
Before Baratie, First Three Arcs showed us Zoro's Skills & Physical Stats, then He had a reality check against Mihawk
From that point onward, each Duel is meant to reveal what's necessary for Zoro to reach Mihawk Level
It's like Oda is using Zoro to tell us what "WSS" actually means
1. His First Fight was against Hatchan & it asked the Question: "What if Zoro fought someone with More Swords?!"
- Before He went to Arlong Park, He only had One-Sword btw
2. His Second Fight was against Mr. 1 & it asked the Question: "What if Zoro fought someone He cannot Cut/Harm?!"
3. His Third Fight was against Ohm & it asked the Question: "What if Zoro fought someone He was unable to reach?!"
What happened during these Fights?
1. Zoro was given Swords by Johnny & Yosaku
2. Zoro learned Breath of All Things & Improved his Cutting Technique
3. Zoro learned Flying Slashes which rely on his Strength
So in these Fights, Oda gave us Three Components that are necessary to Perfect for Zoro to reach WSS
WSS requires Swords , Breath of All Things and Strength
And for each Category, Oda created a "Tool" that we can use to understand someone's Progress in each one
They are Meito Quality, Cutting Technique & Flying Slashes
(This is why Zoro's Sword Style never changes, it's his Swords, Strength & BOAT that are Improving with each Arc)
----------------------------------------------------------------------------------------
Now before we explain Kaku & Ryuma Fights, let's jump to Post-TS.
Similarly, Zoro also had Three Major Fights so far since Post-TS started and as you already said, They clearly Mirror Pre-TS Ones by being related to Land, Sea & Sky. Actually, Oda even went further in showcasing these Parallels by making Zoro Clash with Shark (Hody), Tiger (Fujitora) & Dragon (Kaidou) which obviously not only Sea, Land & Sky but are also Zoro's Animal Themes.
You know what this also means? It means that before Zoro faces Fourth Major Opponent, He will first Clash with Character who represents "Devil" & finally before Mihawk Fight, He will Clash with Character who represents "God or Deva" (Which should be related to Zoro's Promise of making his Name reach even Heavens)
Zoro doesn't necessary Defeat these Five Major Post-TS Villains, but He still prove himself against them by countering what they are Best at
(Zoro was Faster than Hody Underwater, He countered Fujitora's Gravity, He gave Invincible Kaidou a Scar). So expect Zoro to do something Super Epic against Next Two Villains (Without Defeating Them)
----------------------------------------------------------------------------------------
Now, for Post-TS Three Major Fights we've seen so far:
1. His First Fight against Hyouzou asked the question: "What if Zoro fought someone He can't be allowed to take a Hit against?!"
- Zoro was Harmed against Hatchan, Mr. 1, Ohm & Kaku, but against Hyouzou, He can't make such Mistake due to his Deadly Poison
- Oda even used Luffy's Poison Immunity as Hype Tool for this Poison to tell us that Zoro must be Super Careful
2. His Second Fight against Pica asked the question: "What if Zoro fought someone who needs to be Cut multiple times?!" (Endurance Fight)
3. His Third Fight against Kind asked the question: "What if Zoro fought someone & He was on a Time-Limit?!"
What happened during these Fights?
1. Zoro used CoO to make Hyouzou's Attempt at surprising Zoro Futile
2. Zoro used CoA to protect his Swords from this Long Fight & also Pica's Final Attack
3. Zoro unlocked CoC which allowed him to remove Enma's Nerf & also finish Fight before KOH or Mink Drug Effect Ends
So in these Fights, Oda gave us additional Components that must be Perfected for Zoro to reach WSS Level. WSS requires CoO, CoA & CoC.
We could say that Pre-TS showed us Physical Requirements of WSS Level while Post-TS showed Spiritual Requirements
And what's interesting is that they actually complete each other.
Meito Swords are further Enhanced with CoA
Cutting Technique is further Enhanced with CoC
Flying Slashes are further Improved with CoO