Hi jranis,
I’d like to confirm if you want to point the public web site to wix site and keep the mail service at Office 365 and when you try to create an A record in the DNS manager you found there is an un-editable A record have been already existed. If yes, could you please check in the DNS manager what the SharePoint website address is?
As far as I know if you have changed the SharePoint website address to the custom domain, the system will automatically create an un-editable A record and CNAME in DNS manager. However once you change the SharePoint website address to the onmicrosoft domain, those un-editable DNS records will automatically change to editable records, so that you are able to modify or remove them.
Thanks,
Kent Gu