Since you’ve already used Migration Assistant, I’d probably move these items manually rather than run it again just for this. Migration Assistant is better suited to transferring a user account and its data/settings as a whole, not picking out a few individual items afterward. → Transfer to a new Mac with Migration Assistant
First check whether any of this is already synced through iCloud, Google, Exchange, etc. If Calendar, Contacts, or Safari are using the same account on both Macs, signing into that account and enabling those services may be all you need.
For anything stored only on the old Mac:
• Calendar: select each local calendar and use File > Export > Export, then import the resulting .ics file on the new Mac. I’d use individual exports rather than a Calendar Archive, since importing an archive replaces the current calendar data. → Import or export calendars on Mac
• Mail: for mailboxes under On My Mac, select them and choose Mailbox > Export Mailbox. On the new Mac use File > Import Mailboxes > Apple Mail. IMAP/iCloud mail normally comes back when you add the mail account. → Import or export mailboxes in Mail on Mac
• Contacts: if they aren’t synced through an online account, select the contacts and export them as vCards, then import those on the new Mac.
• Safari bookmarks: if Safari isn’t syncing through iCloud, export the bookmarks from Safari on the old Mac and import the bookmarks file into Safari on the new one.
I’d keep the old iMac untouched until you’ve checked that everything you need made it across.
