It's a personal preference on when it's time to upgrade to another model of phone.
It also depends if your current phone is showing signs of wear and tear , isn't performing as it used to do even after clearing the system cache files and things you don't use along with a factory reset if necessary.
Some people just want the next new one which is fine.
The Samsung Galaxy s21 certainly has impressive specs, a good camera , the newest software updates etc ,and the phantom black colour is awesome.
Yes it does not have Sd card support but that can be circumvented via online vaults and there is a plug in that can act as a storage facility.
There are lots of comparison information on tech sites and on YouTube.
Usually I'd also suggest to visit a Samsung Experience Store or Network Highstreet Store to have hands on with a working demo model but most are closed due to various region lockdown rules.
Perhaps a friend , family member or work colleagues have one you could look at.
In regards to choosing a non Samsung phone then again check out tech sites and YouTube and Google search for forums to read posts from owners of the phone's your interested on.
Then there's disposable income if buying sim free or what a network offers via upgrading.
Perhaps create a shortlist of phone's that have caught your attention and start to whittle them down.
In most regions the Samsung Galaxy s20 and s21 comes with a pre fitted screen protector that should ensure no scratches affect the phone.
Personally I remove these plastic versions and apply the Whitestone Dome Glass Screen Protector.
Enjoy your journey.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.