Replace 2 Exchange Servers 2016 & DAG into a single EXCH 2016 server

I was hired as consultant to a company and found they have this setup - checking available licenses, I found they only have a single Exch 2016 STD license.

Instead of moving setup to single server, you can insist them to purchase required license or simply get O365 business / essentials subscription and move everything to O365 and uninstall exchange server, that will save licensing cost and still setup will remain highly available

O365 business essentials / business premium can be used up to 300 users, afterword's you need to get Enterprise subscriptions

I am unable to add the new server to the DAG. Getting:
Server 'MAIL03' has a different OS Version than the other members of the Dag.

Actually the old servers are Windows 2012 Data Center, and the new one is 2016 STD. Is this error because of using 2016 or because it is STD?

How can I overcome this situation?

At the end, and as I mentioned, I will not keep the old servers. Final setup is 1 Exchange Server.

You cannot add exchange server having different OS versions in same DAG

For DAG to work, all servers must be on same OS and same exchange versions

ok, now I am stuck. I want to uninstall the new exchange 2012 STD so I can install it on Windows 2013 Server instead of 2016.
I am getting a very long error (you should be aware of) stating that there are existing mailboxes. I found 2 arbitration mailboxes on the default DB. Trying to move these 2 to another DB so I can uninstall Exchange it gets stuck as "In Progress".
Please advice..

The 2 mailboxes are:
E4E encryption store - active
Microsoft exchange

I will try this:

https://jacob.gardiner-moon.co.uk/2015/11/26/microsoft-exchange-2013-2016-recreate-arbitration-mailboxes/

I finally moved all mailboxes to another new server to be able to uninstall the new Ex server.
Then started the migration normally.