How do I keep my devices and iCloud secure?

What symptoms or problems or issues are you encountering?

How long has this been happening?

What security changes have already been suggested, and what have you then implemented?

[If you don’t trust your spouse, there are other and larger issues awaiting, too.]

First see how many devices are signed in with your Apple ID account Check your Apple Account device list to find where you’re signed in - Apple Support

You can remove other devices that you don't recognise , or may be if your any family member device is also signed in the account , also your Apple ID , its password and all account credentials are known to them that's the reason they can access the account .

There are two methods to secure your iCloud account .

First update your iPhone Update your iPhone or iPad - Apple Support

Take a back up of your precious data How to back up your iPhone, iPad, and iPod touch - Apple Support

Then restore the iPhone to factory settings Restore your iPhone, iPad or iPod to factory settings using a computer - Apple Support

Once again set up new iPhone Set up your iPhone or iPad - Apple Support

And then sign inSign in to your Apple Account - Apple Support with newly created Apple ID and password How to create a new Apple Account - Apple Support to set up iCloud Set up iCloud on all your devices - Apple Support

Turn on iCloud features Change which apps sync and store data with iCloud - Apple Support

if all iCloud features are always turned as on and after that you erase the iPhone , the data is always saved on the iCloud server it will sync when phone is set up .

But the wiser way is take back up of the data as it should not be lost .

The other way is after removing the suspicious devices , change all passwords for Apple ID , the iCloud account credentials , and prefer to use 2FA , you can create a passcode to lock the iPhone .

Read Keep your Apple Account secure on iPhone - Apple Support

Regarding wifi compromising it's impossible , the network is always encrypted , you must create tough to understand password for wifi router / air port express if used , rather change the password for it Change your Wi-Fi network’s name, password and security setting in AirPort Utility on Mac - Apple Support

There is a similar discussion here where I detailed how to secure an account when you feel that someone has access and should not: iPhone 15 Plus Hacked Accounts. Is my iPh… - Apple Community