@Razh Each force field isn't the same in strength. Improving your force field as you become more experience is representative of your will becoming stronger. Why did it improve? You are more tested. That's what will/spirit is.
Ryou is what they refer to Haki and they mostly go for CoA. It isn't a more advanced practice. Mantra isn't a more advanced version of CoO. The people in both places just see it differently mentally.
The warriors of Wano are explained to have strong will/spirit. This is why it's so dramatic to see their will broken. That focus and concentration puts them at an advantage as opposed to someone born in the East Blue that doesn't practice sword fighting as much. There's a pride there.
Zoro living in the East Blue doesn't mine kairoseki, deal with their differing weather, clash from young and is laid to rest with the sword. They are proficient in CoA and practice differently because that is the way of the warrior. It makes them more prepared for Ryuo which is Haki control.
It's one thing to cover yourself with the armor, but to now control it is different thing. It goes back to Law's statement as he first sees Bounce Man in 785.
It's a natural progression to waste the new power, then learn efficiency. Oda showed the aura flow from parts of the body to other sections.
It's not punching harder. There's control, focus for observation, and Haki/spirit itself growing as the character becomes more experienced.
Luffy went through trying to punch harder at 1 point and learned that's not the way. He has to be aware of how he's letting his Haki flow, which is control.
It even fits perfectly because he just grew in awareness. Makes the most sense to jump to control after improving on how he perceives things.
Hardening reinforces the point of punching harder not being the case. The spirit/will can improve, so it can become visible. Haki is there whether it's visible or not.
As Rayleigh said, Haki is in everyone. Becoming a user just mean that you're aware of it and apply it at different levels.
CoO relies on focus, which Kata was really good at. When he wasn't focused, he paid for it. When he was focus, it took Luffy being a more aware version of himself to keep up and get that result.
Characters are not incapable of losing focus. Fighters gain experience and grow in other series.
Predicting the future still falls under awareness. If you can perceive it, it's awareness that the character is experiencing. Especially since he's visualizing it in his mind.
To add on to that, there are times when the character can't see the future as Dogtooth explained. It's hard to predict (862).
Control, predictions, and making your spirit visible are just not complicated. It's actually creative to work all these normal things up together and pair them with the already chaotic DF abilities.