M31s Knox but partially Its there only To secure your Kernel and Bootloader, Your Biometric is not secure or maybe in OneUI Core Samsung didn't written this under Biometric Sections. May be Fixed by Updates
You will get support upto 2024 and You will also get Android 12
LONG TERM USAGE
Device looks and feel modern with Glastic body frame and SAmoled Infinity-O Display.
So for long term usage its probably great. Because your device will secure till 2024. Also you have 25w Adaptive Fast charger with 6000mAH that's good with reverse Power share that's good.