I know to back up my data and cancel subscriptions, etc, but from a privacy and security perspective, are there any additional measures I should consider?
There’s really not much more you can do than what is explained in this article:
Do NOT forget to delete the accounts connected to your Apple ID (eg. If you created PrivacyGuides account with your Apple ID). Because if you forgot even one or two it’s going to be a massive pain to delete (if it’s even possible to recover the login) later
Also, migrating off of iCloud Hide my Email if you were using that to alias before.