Keeping Inbox Mail for longer period of time

There should be no time limit on how long emails remain in your Inbox unless you are using IMAP or Exchange and your email provider has imposed a time limit. Also, many employers impose a 6 month limit on stored emails.

The Apple Mail app does not limit how long emails are kept except in the case of the Trash & Junk folders (for those 2 folders you can set a time limit in Mail > Preferences > Accounts > (select an account) > Mailbox Behaviors.)

Regarding local email retention, in the Apple Mail app you could create a mailbox "On My Mac" and move the Inbox messages you want to keep to that mailbox. You could move them manually or via a rule.

Just be cognizant that if your employer has set an email retention time limit and you save them longer than that limit, you are taking great personal risk vis-à-vis your employer.