I think its still worth remembering the 10(x) is still a 5 year+ old phone that you would be buying of someone you don’t know (different scenario if got it from a family member) it will be at or near the end of recieving IOS updates.
If I can just add to this: The older the phone the more prone it is for something to go wrong or the battery is near it’s EOL requiring further outlay to put it right or the need to buy another phone depending on the issue.
My honest advice is – stick with your current phone whils’t its still useable and save an extra £150ish ontop of the initial “up to” £250 and treat yourself to a new SE 2022 or a good 2022/23 mid range android which will have an Oled screen and decent specs and the latest version of ios/android.
At least you will be the only owner and know how its been treated/not had any work done, along with a manudacturers warranty.
By all means slip your nieces or her husbands phone in your pocket later on 🙂
Exaxtly @AtoB_Dude!