iPhone & iPad not syncing correctly; irritating & continual issue never properly resolved

I have an iPhone 13 (iOS 17.3.1) & an iPad 10 (iOS 17.3.1) and a SINGLE paid for iCloud account with plenty of free space (over 100gb) which I am signed into on each device with the EXACT same log-in.

Some apps are NOT syncing, the most irritating ones being Contacts and Reminders where different versions of their contents exist on each device.

For example, I have recently entered a new contact’s details on my iPad but when that person messaged me their number rather than their name was displayed on my iPhone although their name correctly appeared on my iPad.

An additional issue is that sync between my devices & my wife’s (Reminders in particular) is not working.

Calendars, Messages, Mail and most other apps sync perfectly.

On multiple occasions I have tried resolving this problem by signing out of iCloud on each device, resetting each, then signing back in - as per the regularly repeated advice/instructions. It doesn’t resolve the issue. All settings are mirrored between devices.

Can anyone help with a method of permanently resolving this issue, please??

Your help would be hugely appreciated - I’m tired of the hassle this issue causes!!