The only differences I'd notice on Kitkat would be the dialog has the Holo-white buttons instead of coloured text, there's no FAB in any of the Phone, Contacts or Messages apps, and Peel Remote app has the skeuo UI from Jelly Bean (and won't work without updating to the modern version anyway), and the icons don't have frames.
Back when the S5 was new, this was when Samsung didn't change TouchWiz much across Android versions. I wish they kept that up myself. I don't adjust well to even the most minor change. I'm still upset they removed the splash screen and various whimsical animations from Samsung Health in Android 12. If you try sideloading the older version, it forces an update shortly after. Keeping it offline via NetGuard just breaks the whole Together feature.
I am currently using an A14 5G, hoping that having 5G will get me more than a decade before that network shuts down (I'm sure network shutdowns are always going to be a thing now) and that means I finally have a phone I can keep for some time, but sadly it's an adjustment since modern Samsungs truly don't appeal to me at all, and it takes a ton of effort to "touchwiz-ify" One UI enough to safisfy me. Mainly getting rid of TONS of unnecessary notifications, many which don't allow you to turn them off, that are added to a modern device, and requiring third-party apps like BuzzKill to do the job. Three weeks later and I'm still struggling navigating the blasted thing.