One thing that isn't covered in the guide Enlli posted is data backup/migration as far as I could see.
Although WhatsApp is tied to a number which means you should be good to go if you follow that guide, as you don't want to risk losing your data I'd recommend backing it up regardless. Just be aware that backups can sometimes be a weak spot in your security. That doesn't ever stop me backing up, but I just make sure the backups are as secure as they can be.
Go to WhatsApp > tap More options > Settings > Chats > Chat backup > BACK UP.
You can also backup your WhatsApp data to Google Drive, and that is covered in the WhatsApp documentation. The backups used to be unencrypted, but I believe the option to encrypt that data is now given or even enforced which is ideal if you value privacy.
Pay attention to what app (native or 3rd party, no pun-intended) you are using to send and receive SMS or other text messages, and see what options you have for backing up and restoring that data.
Make sure you know what your iPhone backup consists of, and how easily it can be restored. You don't have to go overboard, but at least knowing that you are able to access all the data (calls, texts, history etc.) no matter what happens can be useful in case anything ever goes wrong.
A tool I find particularly handy is a free 3rd party app which allows you to control, send/receive data (such as SMS and other messages, photos/videos) and there are a few options for both iPhone and Android. Being able to text from my desktop computer is a time-saver. For that reason I also use WhatsApp desktop. I was using the beta version which lets you send and receive messages on up to 5 devices without the need for your phone (or SIM card). That is particularly useful to have in place during a transition such as yours, so you can always have access to your messages, history and be able to send/receive messages and possibly even calls via WhatsApp whilst you are porting your number and you are unable to use your SIM or network.
I also used to use a third party app or service which sent all my texts to my Gmail account. I used it for years, and as a result was able to search any historical text I'd sent or received from any of my past phones via my chosen email client which was handy to keep all my text based communications a lot more organised, and searchable using Google tech, which is pretty good at search.
One thing I've not struggled with is data loss, and that is probably down to security practices and backup plans for my backups and backups and backups. (a single point of failure is not something I'm a fan of, ie. a single backup)
