It seems like OneUI 6 is coming soon. However with the screenshots of the control centre UI, it seems like the 'one handed' UI focus is disappearing as more UI elements are brought higher...
Can't remember last time they released new android version on a flagship without breaking either performance or battery life
That's actually quite normal for most in-place OS upgrades, and isn't specific to Samsung or even Android.
As an old PC nerd, the rule-of-thumb still stands true today: always do a fresh install when doing a major OS upgrade, it'll save you from much agony down the line.