D&D is not married to medievalism. Most people couldn't understand or relate to people from the medieval era, or even imagine it.
D&D is like most popular media married to heroism. And it's not coincidental that quasi-medieval trappings are popular when you are wanting to tell a heroic story. That's because the medieval period was the most recent period in human history where defensive military technology had outstripped offensive military technology. And in periods where defensive technology has the advantage over offensive technology, lone trained warriors with high tech gear are capable of taking on a dozen or two dozen peasants without their high-tech armor at once leading to heroic fiction that extols the virtues of such warriors and captures the imagination.
It's not a coincidence that most "science fiction" that has captured the imagination is actually fantasy in modern dress where this heroic age is recaptured by powered body armor, giant robot suits, personal shields, genetic super-soldiers, or magic given some pseudo-science gloss. It again lets you have a single hero who can plausibly (if you don't think about it too hard) take on dozens of less powerful beings at a time, recreating the heroic narrative. And face it, humans love a good story about a virile human punching a bunch of other less admirable humans.
Realism of any sort gives you cannon fodder and conscripts and trench warfare and horror. Realism gives you artillery randomly squashing you no matter how heroic you are.
I'm not sure you can actually achieve what you want without in some fashion playing a "supers" game. You are literally describing changing the baseline reality to have offensive weaponry more powerful than defensive weaponry, which is exactly the opposite of how you get heroic fiction. Someone has to be bullet proof in some fashion, whether having superspeed or body made of iron or whatnot.