If you had never received any updates before maybe you should change your firmware by flashing your phone but it's a process

You got the phone with one ui 5 then had never received updates or you just chose to not update? If that is so you can update but if you had never received any updates since you bought the phone I don't know but the S21+ is eligible for one ui 7.

If you haven't got it yet it is coming just be patient surely this month you will

Be patient surely by the end of this month you would have it

Battery is bad compared to the previous software but surely they'll also get it fixed just like other devices.