Exactly which phones are they? Depending on the manufacturer some will have an option to export your contacts as a vcf file.
Are you not just using the normal contacts app, in which case your contacts should just be synced to your Google account?
It seems to miss out half my contacts, any idea why?
Check your list of contacts in your web browser on your PC at contacts.google.com or whatever it is, are they all there?
Use this as a lesson. Spend an hour putting them all into Google Contacts at some point. Way easier.I did it old school with a vcf file. All good now
As far as I know , Phone transfer software not only can help you backup Android phone data to computer, but also can help you transfer content from Android to Android.