MerlinBrasil Switching between apps seems tedious and requires multiple swipe attempts to navigate.
I am wondering what you mean by "switching between apps": are you referring to switching between already opened apps, or launching apps from the home screen? If the first, you can glide your finger to the left or right at the very bottom of the screen to switch between them.
It's been a while since I used a Samsung device, so I don't know how it's different on OneUI. Off the top of my head I can't really think how they're making app switching any easier than how I explained above. Would be nice if you could clarify.
MerlinBrasil After installing GrapheneOS, I find that I'm greatly missing the ONE-UI interface.
There simply is no OneUI for GrapheneOS. As far as I'm aware, OneUI is proprietary and has not been integrated into any Android-based operating system other than Samsung's. GrapheneOS stays very close to the Android Open Source Project's UI, which GrapheneOS is based upon. I believe the GrapheneOS developers have made it clear that they don't plan to remake the UI in a significant way, or to deviate from the Android Open Source Project's design in any substantial way. My personal impression is also that, in general, there is not a huge demand for a UI overhaul from this community (with the exception of overhauling the prebundled apps, like the horrible Gallery app).
As has been said, there are multiple apps that can replace the launcher. Perhaps one of them is closer to what you prefer. But they can't give you a UI design beyond the launcher; I don't believe anything other than the operating system are permitted to change that.