Hex installer doesn’t work with one ui 4. Samsung changed the way adb works with the theming so it broke hex.I'd stick with the S21 Ultra. If you want a better UI, download Nova Launcher and a Pixel icon pack. Also #Hex Installer allows you to completely redesign the UI with Pixel style icons and less rounded elements throughout. I'm not sure if it's working with Android 12 yet though. If you hate the bloat, there are ways to disable the majority of it using ADB.
I like the Pixel 6 Pro a lot, but I'm not gonna lie, there are A LOT of things that Samsung phones do better. Samsung has GoodLock for endless customization, Always on Display Customization, the 5G modem works better, and charging seems to be faster and more reliable.