Hello !
You're right : you can't transfer an existing Microsoft 365 subscription from one account to another.
However, you can change your Microsoft 365 Personal subscription to a Family one, simply by upgrading your wife's subscription to Microsoft 365 Family. She will be the owner of the family group, and she won't lose any data.
Learn more about plan upgrading here : https://support.microsoft.com/en-us/office/switch-between-microsoft-365-subscriptions-3fcc1efc-2722-427f-8efa-db94b9b0a36b#bkmk_morefeatures
Your remaining subscriptions days will be converted and added to your Family subscription.
Once you're got the Family subscription, head to your Microsoft account dashboard and share the benefits of your plan. You can share with 5 persons maximum.
Please let me know if you have any other questions !