So when I looked at one of my domains, the simplest one with just two standard user accounts, so neither were Google Groups based email addresses, I generally followed these instructions to consolidate the emails first before deleting the unwanted account which then took care of migrating the Google Drive files and some other stuff (perhaps calendars, recollection is rusty) for the deleted account.
https://www.cubebackup.com/docs/tutorials/transfer-gsuite-data-to-another-account/
Under "Gmail migration" I think you can ignore the comment about upgrading to Google Workspace or G Suite Basic to get Google Workspace data migration if you have a legacy free edition of G Suite. Maybe everyone has already been upgraded to Google Workspace anyway by now and the comment is out of date as I didn't find it necessary to upgrade my free legacy account first.
You should be able to use the above process to either consolidate standard user email accounts or as a process to transfer each to an ordinary free Gmail account. Of course if you consolidate accounts, the one you delete will no longer enable you to receive emails for it unless you set up an alias in one the accounts you are keeping. Unless you can update third parties where the unwanted email addresses have been used it may be an idea to set up an alias for each on an account which will be remaining.
If you are wanting to stay with Google you may also consider consolidating any generic email accounts (
[email protected]) you may have inadvertently created because you didn't understand or avoided use of Google Groups. You can use Google Groups to create any required generic accounts such as
[email protected] and give standard user accounts access to each group as required. Ensure you set up the Group permissions correctly so that the public / anyone can send into the Group but cannot see the posts within the Group. Also set the permissions for your internal user accounts to be able to see the posts within the Group. I think you can do this on a per user basis or set up a permissions group and add relevant users to that and assign the permissions to the permissions group. There is a slight gotcha with Groups though in that I believe you can only migrate one way (in) so be sure it's what you want if you choose this route. The Groups are a feature rather than a separate account so you don't pay for such email addresses. So you could have
[email protected] &
[email protected] plus
[email protected],
[email protected] and because these last two are set up as Groups you'd only actually have two user accounts that relate to specific people to pay for.
The last time I looked it seemed as though you could use a free Gmail account to send email as your domain, somewhere in Gmail settings, inbox settings I think. If this is possible and you have full DNS access on your domain then you may be able to set up email forwarding DNS entries. If that is still possible then that would take care of outgoing emails in the Gmail settings and incoming in the DNS email forwarding settings. This would be the totally free option, save for domain name cost and/or separate DNS service costs if domain isn't hosted somewhere with decent DNS management. The drawback would be that each Gmail account would be independent and not governed under the control of a domain admin.
