I’ve been residing legally in Spain for 5 years but I haven’t requested the permanent residency certificate. Should I apply for permanent residency?

If you currently hold an EU Residency Certificate (green card), you have what you need to benefit from the Withdrawal Agreement to continue living in Spain as you used to. However, obtaining permanent residency has a lot of advantages over the temporary residency certificate, so it is recommended to apply for it once you become eligible.