If one wants a “realistic” sword style one can’t avoid asking “realistic for what?”. Duelling?… Death or pinks? Sport?… Which rules? War?… line up, form shield wall, push, stab, push, stab. Oh you mean broken field confusion? OK go stab guys in the back who are already engaged. Watch your own back, wait for the heavy horse to come and sweep everyone aside. Now line up again and form that shield wall to get some real work done.
If you want real, first figure out what you mean by that, then decide which martial art or sport is going to provide that training to you. Otherwise it’s a craps shoot, join the art with the best hype, hope you learn something you want to learn.
For “real” you may figure it’s best done “freestyle” in a sport like Kendo. But for any sport you need to pay attention to the rules, for any fight you need to pay attention to the rules. Chris Amberger made the point a very long time ago that even duels had rules that were followed. Something that most people would assume had no rules, certainly did. Even warfare has rules.
It’s not only what the rules disallow but what they allow or require that determines the techniques that end up being used. Look at “full contact karate” with the mandatory number of kicks per round, without that rule you’d see boxing. Anybody see a distinguishable martial arts style in the UFC stuff now? I see a very recognizable and distinctive UFC style of fighting but I don’t see anything I’d recognize as a martial art.
The techniques very quickly fit the rules in any contest-dominated art/sport. Kendo looks the way it does because that’s what works.
And Koryu? Is it any more “realistic”? Despite the general idea that it’s all battlefield derived, what I see most is a school that starts out with maybe 4-5 techniques and these get added to over the generations of peace as students want more to do. Students get bored, sensei adds techniques.
Conversely, students get few, sensei gets old, techniques get dropped, modified and simplified so that at least “something” gets left for the next generation to build on. Little of this is tested to see if it’s “real” or not. Dojo storming was never very common, frankly, it’s rude, but perhaps some students did get together out behind the barn to check out the effectiveness of this or that technique. If you’re fast and you use a technique nobody’s seen before, maybe it works, maybe it doesn’t if your opponent has good reactions.
That’s not really the point though, is it? Trying to find a “real” sword art is the same as trying to find that perfect kata that’s unstoppable. If it was out there, we’d all know it I assure you. Same for modern war as for sword, the unstoppable weapon is certainly the atomic bomb but can you, do you, use it? How about just having an overwhelming military that is larger than the next four combined? Certainly it’s unstoppable, but is it useful? It always depends, an unstoppable sword kata would leave your opponent dead. A vast unbeatable military is great if you want to build an empire but just a waste of money if you just want to do a bit of regime change.
Sooner or later, no matter what sort of technique you’ve found, you find something new facing you. A weapon you can’t use isn’t much of a threat. Best to absorb some basic principles and get really good at those, like keeping the shield wall together (have a good aggressive defence), or hit ’em from an unexpected direction (stab them in the back). It’s the basic stuff you fall back on, make sure your basics are solid and you have a good idea what you want to use those basics for.
Then decide which sword school is the most realistic.