Either way you will need to update to firmware 6.X because you cannot link a Nintendo Account without being on that firmware. Which means you're going to need to restore your clean NAND and then update its firmware. From there, you have 2 choices on how you wish to update. The first is using ChoiDujourNX while the second is using a good jig to re-enter RCM after installing the update as detailed in this
tutorial (or if you don't care about fuses, you can just update normally and burn your fuses). Using either of the two methods may warrant a ban however, but there is no way to update your firmware without burning fuses and not risk a ban; they are mutually exclusive.
And linking a Nintendo Account is actually worthwhile because you won't be able to link one after you are banned as well as some games requiring a linked account in order to play them such as the NES app or Pianista.