Here are 3 sharing solutions. You may end up using a mixture of them.
What is Family Sharing? - Apple Support
Contacts are not included in Family Sharing but you can use Airdrop to copy them one by one to another user. See: How to use AirDrop on your iPhone or iPad - Apple Support
You could use a joint email account and keep the contacts on that account. Then, both set up the account on your devices. - How to share Contacts between two Apple I… - Apple Community
To share a lot of contacts: On a computer:
In a web browser log into your Apple Account on iCloud.com and go to the Contacts app.
Export your contacts to a folder on the computer
Log into the other account on iCloud.com
Import the contacts from the folder.
I don't know about third party solutions. Apple keeps non-Apple apps sandboxed so it is hard for them to interface with other software on your device. You might have to keep the contacts in that app instead, and then you'd have problems getting contacts to appear in Apple's apps. I did a one time transfer of most of my wife's contacts and then we do a periodic swap of individual ones with Airdrop.