Hello,
It has been a while that I managed user mailbox archiving (which I assume you refer to), but here is my thought.
1 - some emails should be kept. - Is that 'emails in different user mailboxes, or emails in 1 mailbox? If in 1 put this mailbox in a seperate provisioning group, and set the retention according to requirement. I personally would set the retention to 10 years, and then disable the 'allow deletion' on the archive. Setting it to 'keep items forever' means you can never remove these.
Emails will be archived, and changed into a shortcut, but will obey EV's expiry process.
If you need to keep specific mail from specific mailboxes, you might look into the classification in EV. That is additional license cost, but can assist in determining what specific retention is given to specific mails. As example, all emails are archived with a retention of 2 years, but if an email has in body the word 'merger', set retention to 10 years.